Correctly handle NULL password hashes from the database

synapse/handlers/auth.py

@@ -615,4 +615,7 @@ class AuthHandler(BaseHandler):
         Returns:
             Whether self.hash(password) == stored_hash (bool).
         """
-        return bcrypt.hashpw(password, stored_hash) == stored_hash
+        if stored_hash:
+            return bcrypt.hashpw(password, stored_hash) == stored_hash
+        else:
+            return False