[envtest] Deferred cleanup panic on nil ETCD process state

[Danil-Grigorev]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xd8 pc=0x562ec6147b07]

goroutine 17 [running, locked to thread]:
sigs.k8s.io/controller-runtime/pkg/internal/testing/controlplane.(*Etcd).Stop(...)
	/home/runner/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.21.0/pkg/internal/testing/controlplane/etcd.go:162
sigs.k8s.io/controller-runtime/pkg/internal/testing/controlplane.(*ControlPlane).Stop(0xc00160b348?)
	/home/runner/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.21.0/pkg/internal/testing/controlplane/plane.go:103 +0xe7
sigs.k8s.io/controller-runtime/pkg/envtest.(*Environment).Stop(0xc00160b348)
	/home/runner/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.21.0/pkg/envtest/server.go:211 +0x105
main.Envtest.create.func2(0xc000599610?)

It appears the ETCD cleanup is missing nil check, similar to

controller-runtime/pkg/internal/testing/controlplane/apiserver.go

Line 416 in e8c5c54

if s.processState != nil {

resulting in envtest.Stop() to panic occasionally if the cleanup is performed on setup failure.