fix(video): return none when the av package is unavailable

check_av returned the module on success and False on failure, so the two
callers testing for None treated a failed import as a working av and reached
into it anyway. It now returns None, and the guard that had to test for both
tests for one.

modules/video_models/video_save.py

@@ -205,7 +205,7 @@ def atomic_save_video(
     if metadata is None:
         metadata = {}
     av = check_av()
-    if av is None or av is False:
+    if av is None:
         log.error('Video: ffmpeg/av not available')
         return
     savejob = shared.state.begin('Save video')

modules/video_models/video_utils.py

@@ -54,13 +54,14 @@ def supports_last_frame(model):
 
 
 def check_av():
+    """The av module, or None when it is unavailable; callers guard on the None."""
     install('av')
     try:
         import av
         av.logging.set_level(av.logging.ERROR) # pylint: disable=c-extension-no-member
     except Exception as e:
         log.error(f'av package: {e}')
-        return False
+        return None
     return av