GValues can hold a NULL value

Change-Id: Idc7e4786226e35f8e133429d679964f778e468a8
Marshal GValues that holds a GstBufferingMode value

Change-Id: Id88db168d6c5bd7c2887af88d1be562804915b09
Marshal GValues that holds a GstSchedulingFlags value

Change-Id: Ie04205136a8f5c9d45b7836ac89c3e75ac94ac2e
Marshal GValues that holds a GstContext value

Change-Id: I9c776b47b9999e1d33e5d0323c2c00fc190368ac