Raise value error if a reference string is empty to match behaviour of WER

phone_distance.py

@@ -112,7 +112,7 @@ def phone_error_rate(prediction:str, reference: str, distance_computer:panphon.d
 
         return phone_edits / len(ref_phones)
     else: 
-        return 0.0
+        raise ValueError("one or more references are empty strings")
         
 
 @evaluate.utils.file_utils.add_start_docstrings(_DESCRIPTION, _KWARGS_DESCRIPTION)