miniobject: Add missing (nullable) annotations
miniobject: Add missing (nullable) annotations

gst_mini_object_replace() can take NULL mini-objects.

https://bugzilla.gnome.org/show_bug.cgi?id=730873

mparis/gstreamer,centricular/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,centricular/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,mparis/gstreamer,surround-io/gstreamer,Lachann/gstreamer,surround-io/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,centricular/gstreamer,centricular/gstreamer,mparis/gstreamer