New merge request: Add empty branch state

What does this MR do and why?

A follow-up to !88525 (merged).

This MR adds an empty state to the branch selectors when creating new MRs. It also fixes the padding on the loading icon and deselection not falling back to the default text too.

Screenshots or screen recordings

As @aregnery proposed to add an empty state when no source branch is selected:

Before	After

Screen_Recording_2022-06-13_at_11.43.34

How to set up and validate locally

1. Create a new MR
2. Check source branch has nothing selected and confirm that the empty state is shown
3. Remove the target branch and confirm that the empty state is shown

Related to #361641 (closed)