Update UI copy for merge request tab empty state

Problem to solve

The Compliance Dashboard empty state for merge requests does not provide enough contextual information to understand how this page on the dashboard becomes populated.

Initiated by this discussion in Slack:

Austin Regnery: Why is it that despite there being 20 MR related to this group, nothing shows up for me in the Compliance Dashboard?

Aishwarya: I think we show only merged MRs, but let me confirm quick in the codebase

Matt Gonzales: That's correct 🙂 Most recent, merged MRs; 1 per project

Austin Regnery: Seeing a really easy opportunity to make that clear to novices like me. I am going to open an issue to propose clearer UI copy on empty state 🙂

Conditional logic: Shows only the most recently merged Merge Request; max 1 per project

Current Empty State	Proposed Change (see comment)