[JENKINS-75914] Do not mask empty secrets

Supersedes #377.

The issue encountered in JENKINS-75914 is due to the lack of handling for empty credentials.
A check is performed for normal messages on the console log

I'm adding this check before changing the type of exception, to avoid throwing a MaskedException when there's nothing to mask.

Testing done

I tested this change locally using the example provided in JENKINS-75914 and added a test to cover this case. The test fails without this change.

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