Allow LtiConsumer.instance_guid to be blank.

The instructions say that this should be left blank until the
initial launch, but Django Admin kicks that out because blanks
are not allowed at the model level.

lms/djangoapps/lti_provider/models.py

@@ -26,7 +26,7 @@ class LtiConsumer(models.Model):
     consumer_name = models.CharField(max_length=255, unique=True)
     consumer_key = models.CharField(max_length=32, unique=True, db_index=True)
     consumer_secret = models.CharField(max_length=32, unique=True)
-    instance_guid = models.CharField(max_length=255, null=True, unique=True)
+    instance_guid = models.CharField(max_length=255, blank=True, null=True, unique=True)
 
     @staticmethod
     def get_or_supplement(instance_guid, consumer_key):