[JENKINS-73610] Allow for null values in "ensureFIPS" methods

[jmdesprez]

See JENKINS-73610

Fix the NullPointerException by allowing null value for both ensureFIPSCompliantURIRequest and ensureFIPSCompliantRequest.

Testing done

Existing tests have been amended to include a test with a null parameter, except for AbstractOpenShiftBearerTokenCredentialFIPSTest because a call to getToken without an URL makes no sense.

I also improved the test coverage by adding a test for ensureFIPSCompliantRequest

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