If an app is playing media but hasn't provided a title, say as much instead of "No media playing"

Certain apps, like GNOME's Totem, provide a very rudimentary MPRIS interface with next-to-no
metadata. Currently the applet and lockscreen show "No media playing" when these apps play something, which is
inaccurate. Change the message in case of (playing || paused) && !title to "No title".

BUG: 456516