Test for TimeField not handling empty values

rest_framework/fields.py

@@ -1204,7 +1204,7 @@ class TimeField(Field):
         )
 
         if output_format.lower() == ISO_8601:
-            if (isinstance(value, str)):
+            if isinstance(value, six.string_types):
                 value = datetime.datetime.strptime(value, '%H:%M:%S').time()
             return value.isoformat()
         return value.strftime(output_format)

tests/test_fields.py

@@ -1032,6 +1032,7 @@ class TestTimeField(FieldValues):
     }
     outputs = {
         datetime.time(13, 00): '13:00:00',
+        '00:00:00': '00:00:00',
         None: None,
         '': None,
     }