Return empty string when #message is called

What does this MR do and why?

This started as a quick spike to ferret out where #message is potentially called, by raiseing. Pipeline only failed in one case, proving out that MergeRequestDiffCommit#message isn't really used anywhere on the site currently. Since we are toying with ceasing to store this attribute in the DB, let's turn this MR into an actual test, controlled by a feature flag, so we can disable the value of #message and see what, if anything, breaks; so far, our research points to "nothing".

Related to #520302