allow 'null' to be a value for job tier (Azure#34217)

Author: TajiHarrisMicrosoft

sdk/ml/azure-ai-ml/azure/ai/ml/constants/_job/job.py

@@ -119,6 +119,7 @@ class EntityNames:
         Basic = "basic"
         Standard = "standard"
         Premium = "premium"
+        Null = "Null"
 
     class RestNames:
         Spot = "Spot"
@@ -135,7 +136,7 @@ class RestNames:
 
     REST_TO_ENTITY = {v: k for k, v in ENTITY_TO_REST.items()}
 
-    ALLOWED_NAMES = [EntityNames.Spot, EntityNames.Basic, EntityNames.Standard, EntityNames.Premium]
+    ALLOWED_NAMES = [EntityNames.Spot, EntityNames.Basic, EntityNames.Standard, EntityNames.Premium, EntityNames.Null]
 
 
 class JobPriorityValues: