Update scores/llr.py

scores/llr.py

@@ -10,7 +10,7 @@ class LLR(ScoreBasis):
 
     def windowed_scoring(self, audios, score_rate):
         if len(audios) != 2:
-            raise ValueError('LLR needs a reference and a test signals.')
+            return None
         return cal_LLR(audios[0], audios[1], score_rate)
 
 def cal_LLR(ref_wav, deg_wav, srate):