Make a mount point of "/" work correctly
As far as I can tell, this is neither explicitly allowed nor forbidden by RFC 7826.
Meanwhile, URLs such as rtsp://:554 or rtsp://:554/ are in use in the wild (presumably with non-GStreamer servers).
GStreamer's prior behavior was confusing, in that gst_rtsp_mount_points_add_factory() would appear to accept a mount path of "" or "/", but later connection attempts would fail with a "media not found" error.
This commit makes a mount path of "/" work for either form of URL, while an empty mount path ("") is rejected and logs a warning.