tests: Update audio test to recognize missing sounds as NullAudioSound

tests/audio/test_loading.py

@@ -1,6 +1,5 @@
 import pytest
 
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 def test_missing_file(audiomgr):
     sound = audiomgr.get_sound('/not/a/valid/file.ogg')
-    assert sound is None
+    assert str(sound).startswith('NullAudioSound')