Render call to action on MR empty state (rapid diffs)
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### Summary
We should render a call to action with the correct text on the rapid diffs empty state (MR page).
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The following discussion from !186255 should be addressed:
- [ ] @jerasmus started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/186255#note_2426182740):
> **note:** The secondary text + button need to conditionally render based on where the empty state is rendered. To keep changes to a minimum in this MR, I'm thinking we can add the secondary text and button in a follow-up MR/issue.
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