Close popup label menu after return pressed on empt input

Problem to solve

Closing popup Labels/Select Labels is done with Escape key or "x" mark via mouse click.

The Return/Enter key press when input is empty could close the menu.

The use scenario is this:

1. One types new label name and presses enter to add it
2. The input is cleared for adding new label.
3. Repeats for every label
4. After last label added input is again empty
5. Pressing Return/Enter could be last "command" in flow of adding labels (closing the Labels menu)

Intended users

Users of gitlab using mostly keyboard to navigate