revert no_null check

Author: sycai

packages/bigframes/tests/system/small/bigquery/test_ai.py

@@ -342,7 +342,7 @@ def test_ai_if_multi_model(session, bq_connection):
 
     result = bbq.ai.if_((df["image"], " contains an animal"))
 
-    assert len(result) == len(df)  # nulls are allowed
+    assert _contains_no_nulls(result)
     assert result.dtype == dtypes.BOOL_DTYPE
 
 
@@ -364,7 +364,7 @@ def test_ai_classify_multi_model(session, bq_connection):
 
     result = bbq.ai.classify(df["image"], ["photo", "cartoon"])
 
-    assert len(result) == len(df)  # nulls are allowed
+    assert _contains_no_nulls(result)
     assert result.dtype == dtypes.STRING_DTYPE