Update scores/ssnr.py

scores/ssnr.py

@@ -8,7 +8,7 @@ class SSNR(ScoreBasis):
 
     def windowed_scoring(self, audios, score_rate):
         if len(audios) != 2:
-            raise ValueError('SSNR needs a reference and a test signals.')
+            return None
         return cal_SSNR(audios[0], audios[1], score_rate)
 
 def cal_SSNR(ref_wav, deg_wav, srate=16000, eps=1e-10):