Can't convert Traktor files where one or more entries are without a location, or the location file is blank

[alza-bitz]

From a user submitted error report, it would appear that Traktor allows entries without a location, or entries with a blank location file (I don't know why Traktor allows this).

I'm not sure it makes sense to convert these entries, by also allowing downstream items (and consequently Rekordbox tracks) without a location, by making it optional everywhere.. I'd rather make location optional for Traktor, and leave it required everywhere else (until I'm proved otherwise).

This can be handled by filtering the collection to only those entries with a location and a non-blank location file (other entries won't be converted).

The user submitted Traktor collection only had a few entries in ~32,000 without a location or a blank location file, so this seems sensible (although I only have one example to work from!)