Improve HandleAdmission resiliency

[sslavic]

What this PR does / why we need it:

While testing ingress-nginx v0.40.1 on Kubernetes v1.17.12, with extensions/v1beta1 Ingress, somehow nil ended up being handed over to HandleAdmission, and controller panicked.

2020/10/05 15:50:37 http: panic serving 10.6.1.61:35040: runtime error: invalid memory address or nil pointer dereference
goroutine 344 [running]:
net/http.(*conn).serve.func1(0xc00017ca00)
	net/http/server.go:1801 +0x147
panic(0x1941ce0, 0x28d4200)
	runtime/panic.go:975 +0x3e9
k8s.io/ingress-nginx/internal/admission/controller.(*IngressAdmission).HandleAdmission(0xc000519c10, 0x1d9aa00, 0xc000a21620, 0x600, 0x0, 0x0, 0x0)
	k8s.io/ingress-nginx/internal/admission/controller/main.go:68 +0x91
k8s.io/ingress-nginx/internal/admission/controller.(*AdmissionControllerServer).ServeHTTP(0xc000519c20, 0x1dd23c0, 0xc000102380, 0xc000124900)
	k8s.io/ingress-nginx/internal/admission/controller/server.go:82 +0x24c
net/http.serverHandler.ServeHTTP(0xc000103960, 0x1dd23c0, 0xc000102380, 0xc000124900)
	net/http/server.go:2843 +0xa3
net/http.(*conn).serve(0xc00017ca00, 0x1dd5d40, 0xc0000cdcc0)
	net/http/server.go:1925 +0x8ad
created by net/http.(*Server).Serve
	net/http/server.go:2969 +0x36c

This PR at least makes the controller more resilient. PR also adds a test case that extensions/v1beta1 validation is not supported / gets rejected; migration to networking.k8s.io/v1beta1 or networking.k8s.io/v1 is advised instead.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

Hi @sslavic. Thanks for your PR.

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[aledbf]

/ok-to-test

[aledbf]

/lgtm
/approve

[aledbf]

@sslavic thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, sslavic

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