player: subtitle URI may be NULL
player: subtitle URI may be NULL

When the media URI is changed, but not the subtitle URI, the playbin cannot
know if it should ignore the subtitles, the user shall inform the playbin that
the subtitle is NULL. But gstplayer API forbid us to do that. This one-liner
patch enables it.

amorenoc/gst-player,shakin/gst-player,jabiers/gst-player,JimmyOhn/gst-player,codomania/gst-player,jabiers/gst-player,shakin/gst-player,sdroege/gst-player,JimmyOhn/gst-player,JimmyOhn/gst-player,justinjoy/gst-player,sdroege/gst-player,shakin/gst-player,sdroege/gst-player,codomania/gst-player,amorenoc/gst-player,JimmyOhn/gst-player,amorenoc/gst-player,amorenoc/gst-player,codomania/gst-player,codomania/gst-player,jabiers/gst-player,JimmyOhn/gst-player,justinjoy/gst-player,amorenoc/gst-player,guiseco/gst-player,guiseco/gst-player,codomania/gst-player,sdroege/gst-player,sdroege/gst-player,justinjoy/gst-player,guiseco/gst-player