Panic fix in generation.go

[NoSkillGirl]

Signed-off-by: NoSkillGirl singhpooja240393@gmail.com

Related issue

Fixed generation.go interface conversion panic:

panic: interface conversion: interface {} is nil, not map[string]interface {}
goroutine 17228 [running]:
github.com/kyverno/kyverno/pkg/webhooks.stripNonPolicyFields(0xc0056462d0, 0xc0011c6b40, 0x211b700, 0xc0011c6960, 0x4, 0xc00597ab57)
	/Users/shutingzhao/go/src/github.com/realshuting/kyverno/pkg/webhooks/generation.go:191 +0xfd9
github.com/kyverno/kyverno/pkg/webhooks.(*WebhookServer).handleUpdateTargetResource(0xc000b13c80, 0xc00010f520, 0xc0038f64a8, 0x1, 0x1, 0xc0011c6ab0, 0x211b700, 0xc0011c6960)
	/Users/shutingzhao/go/src/github.com/realshuting/kyverno/pkg/webhooks/generation.go:165 +0x6af
github.com/kyverno/kyverno/pkg/webhooks.(*WebhookServer).handleUpdate(0xc000b13c80, 0xc00010f520, 0xc0038f64a8, 0x1, 0x1)
	/Users/shutingzhao/go/src/github.com/realshuting/kyverno/pkg/webhooks/generation.go:110 +0x4c5
github.com/kyverno/kyverno/pkg/webhooks.(*WebhookServer).HandleGenerate(0xc000b13c80, 0xc00010f520, 0xc0038f64a8, 0x1, 0x1, 0xc0000d9500, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/shutingzhao/go/src/github.com/realshuting/kyverno/pkg/webhooks/generation.go:92 +0xd86
created by github.com/kyverno/kyverno/pkg/webhooks.(*WebhookServer).ResourceMutation
	/Users/shutingzhao/go/src/github.com/realshuting/kyverno/pkg/webhooks/server.go:379 +0xdb3
rpc error: code = Unknown desc = Error: No such container: e7aa7f0473911a3cb684d1037bb228df90908449c44d13172cecb8b190d0ade5%

What type of PR is this?

/kind bug

Proposed changes

Added if condition before conversion.

Checklist

• I have read the contributing guidelines.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[realshuting]

This can be improved to:

if _, found := obj["metadata"]; found {
	requiredMetadataInObj := make(map[string]interface{})
	if annotations, found := obj["metadata"].(map[string]interface{})["annotations"]; found {
		requiredMetadataInObj["annotations"] = annotations
        }
}