fix is null query

server/djrandom/fingerprint/fingerprint.py

@@ -31,7 +31,7 @@ class Fingerprinter(object):
     def compute_fingerprints(self, run_once):
         """Compute fingerprints of new files."""
         while True:
-            mp3 = MP3.query.filter(MP3.echoprint_fp.null()
+            mp3 = MP3.query.filter(MP3.echoprint_fp == None
                                    ).limit(1).first()
             if not mp3:
                 if run_once: