handle NULL albums and artists in hashing functions
handle NULL albums and artists in hashing functions

When the album and artist hashes are generated, we make sure not
to add entries with NULL albums or artists to the hashes since it
doesn't make sense. However, when subsequently using these hashes,
it's more convenient if we can call them with NULL artists/albums.

hyperair/libgpod,hyperair/libgpod,hyperair/libgpod,neuschaefer/libgpod,neuschaefer/libgpod,neuschaefer/libgpod,hyperair/libgpod,neuschaefer/libgpod,hyperair/libgpod,neuschaefer/libgpod