Update models.py

Minor style fix

caravel/models.py

@@ -428,10 +428,8 @@ class Database(Model, AuditMixinNullable):
             if self.sqlalchemy_uri.startswith(db_type):
                 return grains
 
-    def dttm_converter(self, dttm, tf=None):
+    def dttm_converter(self, dttm, tf='%Y-%m-%d %H:%M:%S.%f'):
         """Returns a string that the database flavor understands as a date"""
-        if tf is None or tf == '':
-            tf = '%Y-%m-%d %H:%M:%S.%f'
         default = "'{}'".format(dttm.strftime(tf))
         iso = dttm.isoformat()
         d = {