Resolve TimeField representation for midnight value. (#4107)

rest_framework/fields.py

@@ -1191,7 +1191,7 @@ class TimeField(Field):
         self.fail('invalid', format=humanized_format)
 
     def to_representation(self, value):
-        if not value:
+        if value in (None, ''):
             return None
 
         output_format = getattr(self, 'format', api_settings.TIME_FORMAT)

tests/test_fields.py

@@ -1063,7 +1063,8 @@ class TestTimeField(FieldValues):
         '99:99': ['Time has wrong format. Use one of these formats instead: hh:mm[:ss[.uuuuuu]].'],
     }
     outputs = {
-        datetime.time(13, 00): '13:00:00',
+        datetime.time(13, 0): '13:00:00',
+        datetime.time(0, 0): '00:00:00',
         '00:00:00': '00:00:00',
         None: None,
         '': None,