Returning none insted of empty FrCont in preproc Wrapper

bob/bio/video/preprocessor/Wrapper.py

@@ -136,6 +136,9 @@ class Wrapper(bob.bio.base.preprocessor.Preprocessor):
                 # add image to frame container
                 fc.add(index, preprocessed, quality)
 
+        if not len(fc):
+          return None
+
         return fc
 
 

bob/bio/video/test/test_preprocessor.py

@@ -77,6 +77,11 @@ def test_missing_annotations():
     assert preprocessed_video[0][1] == 3
     assert [x[0] for x in preprocessed_video] == ['0', '1', '3']
 
+    annotations = {}
+    preprocessed_video = preprocessor(video, annotations)
+
+    assert preprocessed_video is None
+
 
 def test_detect():