[mono] Return null if Podcasts playlist is null
[mono] Return null if Podcasts playlist is null

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