[Fix] handle metadata=null on service-account keys

Addresses Greptile P1: `metadata: null` on a key with a reserved field
was crashing inside prepare_metadata_fields because cast(dict, None) is
a runtime no-op, so the loop hit `reserved_field in None` and returned
500 instead of 400.

Extend the rejection condition to cover `casted_metadata is None`, so
attempts to clear an immutable field via `metadata: null` return 400
consistently with the overwrite path. Non-service-account keys still
fall through to the existing "clear all metadata" behavior.

litellm/proxy/management_endpoints/key_management_endpoints.py

@@ -1523,7 +1523,7 @@ def prepare_metadata_fields(
         existing_value = existing_metadata.get(reserved_field)
         if existing_value is None:
             continue
-        if (
+        if casted_metadata is None or (
             reserved_field in casted_metadata
             and casted_metadata[reserved_field] != existing_value
         ):

tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py

@@ -1300,6 +1300,25 @@ async def test_update_rejects_explicit_null_service_account_id():
     assert exc_info.value.status_code == 400
 
 
+@pytest.mark.asyncio
+async def test_update_rejects_whole_metadata_null_on_service_account_key():
+    """
+    `metadata: null` on a key with a reserved field would have dereferenced None
+    inside the reserved-field loop and returned a 500. Must surface as a 400
+    since the effect would be to clear an immutable field.
+    """
+    data = UpdateKeyRequest(key="sk-1", metadata=None, team_id="IJ")
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-123"},
+    )
+
+    with pytest.raises(HTTPException) as exc_info:
+        await prepare_key_update_data(data=data, existing_key_row=existing_key)
+    assert exc_info.value.status_code == 400
+
+
 @pytest.mark.asyncio
 async def test_update_without_metadata_still_preserves_existing():
     """Omitting metadata entirely must not drop existing metadata fields."""