Fix empty arguments in the cheat sheet

What does this MR do?

While going through the Rails cheat sheet, I've noticed several cases when no example arguments were provided:

p = Project.find_by_full_path('')
u = User.find_by_username('')

It looks confusing to me, and I think it would be better to provide some kind of a sample username / project path / etc there.

This MRs fixes all cases with empty arguments ('' or "") on this page.