Only auto-fill coordinates for non-empty location strings

When editing the location string we try to be smart and automatically add
the correct coordinates (assuming we have a location of this name already
in the dive list). So if you return to the same dive spot you'll get the
correct coordinates by default. But this creates bogus result if we allow
an empty location to be matched, as it makes no sense to assume that all
dives without a location name were at the same coordinates.

Fixes #498

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>