Dropdown widget - honour the "Empty facet behavior"

Patch with the parent's test (improved code but same in principle), to see it fail.

Andras_Szilagyi → credited vengador → .

Andras_Szilagyi → credited escuriola → .

Gave it a good round of testing and new empty behaviour is well covered in phpunit testing. I also credited the devs from the parent issue.

Automatically closed - issue fixed for 2 weeks with no activity.