JENKINS-76080 - Validate signatures fix when secrets are not set

[HybridProgrammer]

Created tests with @Issue("JENKINS-76080") annotation to better understand the behavior described by users. I was able to recreate the issue when a shared secret config was created in jenkins but no share secret jenkins credential was created or selected. When the payload is sent to /github-webhook/ endpoint without Github's X-Hub-Signature headers the HTTP 400 error is returned.

Testing done

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