Fix nil pointer dereference

[mlajkim]

Description

We encountered a bug while attempting to recover from a panic in the Athenz webhook: a runtime error occurred due to an invalid memory address or nil pointer dereference.

Also added temporary unit test just in case.

Type of change

• Bug fix
• New feature
• Refactoring (no functional changes, no api changes)
• Non-code changes (update documentation, pipeline, etc.)

Flags

• Breaks backward compatibility
• Requires a documentation update
• Has untestable code

Related issue/PR

• Fixes Handles both requests v1beta1 and v1 of api/authorization #22

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Checklist

• Followed the guidelines in the CONTRIBUTING document
• Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• Tested and linted the code
• Commented the code
• Made corresponding changes to the documentation
• Passed all pipeline checking

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip]
• Delete the branch after merge