Fix unecessary and ill-timed MR emails

Overview

There are a few issues being observed with MR emails.

Issue 1 - No commit email

I am fine with that, because I chose to create an empty MR (and it's WIP anyway).

Should we restrict this email to conflicts-only @lulalala?

Issue 2 - "Source branch is missing" email after merge

(See https://gitlab.com/gitlab-org/gitlab-ce/issues/48409)

Sometimes after merging an MR, an email is sent saying the MR cannot be merged.