preset: don't return empty preset lists
preset: don't return empty preset lists

Add a shortcut for the cases where an element implements the preset iface but
has no presets and return NULL instead of an empty list in that case.

jpxiong/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,jpxiong/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,mparis/gstreamer,mparis/gstreamer,krichter722/gstreamer,centricular/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer