fix: skip syncing controlplane's infrastructure templates for EKS Clusterclass

[supershal]

What problem does this PR solve?:
This bug was found when integrating EKS provider with NKP UI.
The Managed Kubernetes providers like EKS and AKS will not have controlplane infrastructure provider reference.
This PR copies controlplane infrastructure provider template only if they exists.
Tested with unit tests:
Before fix the test failed with:

Starting the test environment manager
I1027 17:01:27.066340    8612 controller.go:204] "Starting EventSource" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace" source="kind source: *v1beta1.ClusterClass"
I1027 17:01:27.066340    8612 controller.go:204] "Starting EventSource" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace" source="kind source: *v1.Namespace"
I1027 17:01:27.169218    8612 controller.go:239] "Starting Controller" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace"
I1027 17:01:27.169236    8612 controller.go:248] "Starting workers" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace" worker count=1
E1027 17:01:29.548837    8612 controller.go:347] "Reconciler error" err="failed to copy source ClusterClass source/test-5r8wr or its referenced Templates to namespace target-kx6ls: error processing references: failed to get reference: failed to get GenericBootstrapConfigTemplate test-5r8wr/source: failed to retrieve GenericBootstrapConfigTemplate source/test-5r8wr: GenericBootstrapConfigTemplate.bootstrap.cluster.x-k8s.io \"test-5r8wr\" not found" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace" Namespace="target-kx6ls" namespace="" name="target-kx6ls" reconcileID="8ca531d5-2634-4a4c-9b96-fe34a9245399"
E1027 17:01:29.554044    8612 controller.go:347] "Reconciler error" err="failed to copy source ClusterClass source/test-5r8wr or its referenced Templates to namespace target-kx6ls: error processing references: failed to get reference: failed to get GenericInfrastructureClusterTemplate test-5r8wr/source: failed to retrieve GenericInfrastructureClusterTemplate source/test-5r8wr: GenericInfrastructureClusterTemplate.infrastructure.cluster.x-k8s.io \"test-5r8wr\" not found" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace" Namespace="target-kx6ls" namespace="" name="target-kx6ls" reconcileID="840dc3e9-594e-43f1-83c4-ab549d8c11f1"
E1027 17:01:29.562716    8612 signal_unix.go:925] "Observed a panic" controller="syncclusterclass" controllerGroup="" controllerKind="Namespace" Namespace="target-87wdz" namespace="" name="target-87wdz" reconcileID="fb7cbaa9-cf65-4cce-87b3-d773403de9d2" panic="runtime error: invalid memory address or nil pointer dereference" panicGoValue="\"invalid memory address or nil pointer dereference\"" stacktrace=<
	goroutine 298 [running]:
	k8s.io/apimachinery/pkg/util/runtime.logPanic({0x10299b1d0, 0x1400099bb00}, {0x1026c5800, 0x103940090})
		/Users/shalinpatel/go/pkg/mod/k8s.io/apimachinery@v0.32.9/pkg/util/runtime/runtime.go:107 +0x94
	sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Reconcile.func1()
		/Users/shalinpatel/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.20.4/pkg/internal/controller/controller.go:108 +0xdc
	panic({0x1026c5800?, 0x103940090?})
		/Users/shalinpatel/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.1.darwin-arm64/src/runtime/panic.go:783 +0x120
	github.com/nutanix-cloud-native/cluster-api-runtime-extensions-nutanix/pkg/controllers/namespacesync.walkReferences({0x10299b1d0, 0x1400099bb00}, 0x140009e6008, 0x14000ff3aa0)
		/Users/shalinpatel/gitrepos/cluster-api-runtime-extensions-nutanix/pkg/controllers/namespacesync/references.go:63 +0x44
	github.com/nutanix-cloud-native/cluster-api-runtime-extensions-nutanix/pkg/controllers/namespacesync.copyClusterClassAndTemplates({0x10299b1d0, 0x1400099bb00}, {0x14b31e140, 0x14001082f30}, {0x14d490230, 0x14001083a70}, 0x14000ff3b38?, {0x14001100340, 0xc})
		/Users/shalinpatel/gitrepos/cluster-api-runtime-extensions-nutanix/pkg/controllers/namespacesync/copy.go:25 +0xb4
	github.com/nutanix-cloud-native/cluster-api-runtime-extensions-nutanix/pkg/controllers/namespacesync.(*Reconciler).Reconcile(0x14000e53800, {0x10299b1d0, 0x1400099bb00}, {{{0x0?, 0x14000ff3c38?}, {0x14001100340?, 0x14000ff3c58?}}})
		/Users/shalinpatel/gitrepos/cluster-api-runtime-extensions-nutanix/pkg/controllers/namespacesync/controller.go:134 +0x134
	sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Reconcile(0x140005e1240?, {0x10299b1d0?, 0x1400099bb00?}, {{{0x0?, 0x0?}, {0x14001100340?, 0x0?}}})
		/Users/shalinpatel/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.20.4/pkg/internal/controller/controller.go:119 +0x8c
	sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler(0x1029b3160, {0x10299b208, 0x14000035270}, {{{0x0, 0x0}, {0x14001100340, 0xc}}}, 0x0)
		/Users/shalinpatel/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.20.4/pkg/internal/controller/controller.go:334 +0x280
	sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem(0x1029b3160, {0x10299b208, 0x14000035270})
		/Users/shalinpatel/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.20.4/pkg/internal/controller/controller.go:294 +0x16c
	sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func2.2()
		/Users/shalinpatel/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.20.4/pkg/internal/controller/controller.go:255 +0x74
	created by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func2 in goroutine 313
		/Users/shalinpatel/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.20.4/pkg/internal/controller/controller.go:251 +0x4e4

Which issue(s) this PR fixes:
Fixes #

How Has This Been Tested?:

Special notes for your reviewer:

[dkoshkin]

Thank you for the tests!

[dlipovetsky]

Thanks for finding this bug!

[dlipovetsky]

Brilliant! Thank you 🙏