Fix custom inventory scripts on org deletion

We'll now clear the organization for it, this requires a database
migration to accept null values for custom inventory scripts

awx/main/models/inventory.py

@@ -1281,7 +1281,9 @@ class CustomInventoryScript(CommonModelNameNotUnique):
         'Organization',
         related_name='custom_inventory_scripts',
         help_text=_('Organization owning this inventory script'),
-        on_delete=models.CASCADE,
+        blank=False,
+        null=True,
+        on_delete=models.SET_NULL,
     )
 
     def get_absolute_url(self):