[fix] Fix immediate_downstream/upstream UNSET assertions in lineage tests

Change `is None` to `not` for immediate_downstream and immediate_upstream
checks to handle msgspec UNSET values in v9 lineage test assertions.

Made-with: Cursor

Author: Aryamanz29

tests_v9/integration/lineage_test.py

@@ -519,7 +519,7 @@ def test_fetch_lineage_start_list_detailed(
     assert results[4].guid == view.guid
     assert results[4].immediate_upstream and len(results[4].immediate_upstream) == 1
     assert results[4].immediate_upstream[0].guid == mview.guid
-    assert results[4].immediate_downstream is None
+    assert not results[4].immediate_downstream
     lineage = FluentLineage(
         starting_guid=table.guid,
         direction=LineageDirection.UPSTREAM,
@@ -529,8 +529,8 @@ def test_fetch_lineage_start_list_detailed(
     assert response
     assert not response.has_more
     assets = response.current_page()
-    assert assets[0].immediate_upstream is None
-    assert assets[0].immediate_downstream is None
+    assert not assets[0].immediate_upstream
+    assert not assets[0].immediate_downstream
 
 
 def test_fetch_lineage_middle_list(