fix: #114 description should be optional

Author: enriquezrene

time_tracker_api/time_entries/time_entries_namespace.py

@@ -35,7 +35,7 @@
         description='The id of the selected activity',
         example=faker.uuid4(),
     ),
-    'description': fields.String(
+    'description': NullableString(
         title='Comments',
         required=False,
         description='Comments about the time entry',