[Bug 789319] Fix up GIR annotations

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=789319

--- Comment #1 from Arun Raghavan <arun at arunraghavan.net> ---
Created attachment 362046
  --> https://bugzilla.gnome.org/attachment.cgi?id=362046&action=edit
devicemonitor: Return NULL instead of FALSE

Same effect, meaning is clearer.

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