use .IsSet() instead if .Get() != NULL

Change-Id: Ia2b7a719fd398e78cc3b11d4f7b02cb81179f65f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3488
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

HaikuDepot: Lists; Categories + Stabilities

Remove use of custom list class where it is not
really required in the area of Categories. Also
introduces stabilities relayed over from the
Server and used in the HD user interface instead
of being hard-coded.

Relates To #15534

Change-Id: Ib71141e71cd4a0b4882827e2e59b62072de01b4b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3331
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>