Warn on empty match

[anlandu]

Describe the solution you'd like
Was chatting with @JaydipGabani about how it's pretty easy for unsuspecting users to create a mutation (or constraint, for that matter) with no match section, which would mutate everything and very likely take down the cluster. Curious if we've ever considered adding a warning in our constraint/mutation validation if match is empty? Or if particular fields within match (apiGroups/kinds) are empty? Not pressing but we thought it would be interesting to start the conversation.

Environment:

• Gatekeeper version: *
• Kubernetes version: (use kubectl version): *