MBS-6900: Make sure the empty field parses to the empty coordinate value

The Field::Coordinates field now sets validate_when_empty to true
so 'validate' is called even when the field is empty. The empty
string is a validate entry for this field, and parses to undefined
longitude and latitude.
Merge remote-tracking branch 'ocharles/mbs-6900' into beta

* ocharles/mbs-6900:
  MBS-6900: Make sure the empty field parses to the empty coordinate value

Conflicts:
	t/lib/t/MusicBrainz/Server/Form/Field/Coordinates.pm