ext/gconf/: Ignore changing the GConf key to "". Ignore GConf key updates that don't actually change the string.

Original commit message from CVS:
* ext/gconf/gconf.c: (gst_gconf_get_default_audio_sink),
(gst_gconf_get_default_video_sink),
(gst_gconf_get_default_audio_src),
(gst_gconf_get_default_video_src):
* ext/gconf/gconf.h:
* ext/gconf/gstgconfaudiosink.c: (gst_gconf_audio_sink_reset),
(gst_gconf_audio_sink_init), (gst_gconf_audio_sink_dispose),
(do_toggle_element):
* ext/gconf/gstgconfaudiosink.h:
* ext/gconf/gstgconfaudiosrc.c: (gst_gconf_audio_src_reset),
(gst_gconf_audio_src_init), (gst_gconf_audio_src_dispose),
(do_toggle_element):
* ext/gconf/gstgconfaudiosrc.h:
* ext/gconf/gstgconfvideosink.c: (gst_gconf_video_sink_reset),
(gst_gconf_video_sink_init), (gst_gconf_video_sink_dispose),
(do_toggle_element):
* ext/gconf/gstgconfvideosink.h:
* ext/gconf/gstgconfvideosrc.c: (gst_gconf_video_src_reset),
(gst_gconf_video_src_init), (gst_gconf_video_src_dispose),
(do_toggle_element):
* ext/gconf/gstgconfvideosrc.h:
Ignore changing the GConf key to "". Ignore GConf key updates
that don't actually change the string.
For now, ignore the GConf key when the state is > READY, as
it breaks streaming. Sometime it will be nice to bring the
new sink online even mid-stream, by sending NEWSEGMENT info
and possibly prerolling.
(Fixes #326736)