By default, save job_created as null in db

* Want to avoid saving date as empty string
* Should default to null so that it's obvious the field is empty

awx/main/migrations/0124_event_partitions.py

@@ -84,6 +84,6 @@ class Migration(migrations.Migration):
         FakeAddField(
             model_name='jobevent',
             name='job_created',
-            field=models.DateTimeField(editable=False),
+            field=models.DateTimeField(null=True, editable=False),
         ),
     ]

awx/main/models/events.py

@@ -483,6 +483,7 @@ class JobEvent(BasePlaybookEvent):
         editable=False,
     )
     job_created = models.DateTimeField(
+        null=True,
         editable=False
     )