Fixed 500 internal server error for MRs with missing source/target in search results

This is to address GitHub issue 7033.

The problem is that when a project has been deleted, merge_request.source_project or merge_request.target_project can be nil, which would cause undefined method 'name_with_namespace' for nil:NilClass when the aforementioned method is called without checks, as is done in _merge_request.html.haml.

All I did was check for it inline using a ternary expression, and if a source/target project is nil, it uses <em>removed project</em> instead. I'm not a huge fan of returning html from ruby that's within haml (or styling with <em> instead of CSS); I was going for a small/unobtrusive fix, but would really appreciate suggestions on a cleaner fix.