omx: allow 0 feature

omx: allow 0 feature

Previously the omx plugin was blacklisted if GST_OMX_CONFIG_DIR
points to an invalid path or if the gstomx.conf contains 0 valid
component.
Problem is that once the plugin is blacklisted, a rescan is not
triggered upon changes of the env var or the gstomx.conf file
despite being setup with gst_plugin_add_dependency.

This also makes it more consistent with other plugins that auto
generate features. For example gst-{ffmeg,libav}, gstreamer-vaapi,
v4l2 video dec.

To clarify the diff, the plugin_init func will return TRUE even if
g_key_file_get_groups returns 0 element and even if
g_key_file_load_from_dirs fails.

https://bugzilla.gnome.org/show_bug.cgi?id=782867