[com_fields] empty dropdown if no groups are created.

[C-Lodder]

Steps to reproduce the issue

When adding a new field, you have an option to assign it to a group. If no groups have been created, the dropdown still exists but has no options.

Expected result

A message should be displayed instead of a empty dropdown, saying something along the lines of:

Please create a group if you wish to assign the field to one

Actual result

Empty dropdown

System information (as much as possible)

Joomla 3.7-aplha 1

[Bakual]

In staging, it already looks different:

That should fix that issue as well, right?

[zero-24]

Should be fixed in staging with #13155 ;)