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

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