fix: avoid invalid CAST(NULL AS NULL) in SQLGlot compiler (#17487)

This PR resolves a compilation crash caused by the SQLGlot compiler
attempting to generate an invalid CAST(NULL AS NULL) statement in
BigQuery, which triggers a syntax/validation error (e.g., Unexpected
keyword NULL).

Fixes #<524701452> 🦕

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: shuoweil

packages/bigframes/bigframes/core/compile/sqlglot/sql/base.py

@@ -69,6 +69,8 @@ def literal(value: typing.Any, dtype: dtypes.Dtype | None = None) -> sge.Express
         return sge.Null()
 
     if value is None:
+        if str(sqlglot_type).upper() == "NULL":
+            return sge.Null()
         return cast(sge.Null(), sqlglot_type)
     if dtypes.is_struct_like(dtype):
         items = [

packages/bigframes/tests/unit/core/compile/sqlglot/sql/test_base.py

@@ -159,3 +159,15 @@ def test_literal_explicit_dtype(value, dtype, expected):
 def test_literal_for_list(value: list, expected: str):
     got = sql.to_sql(sql.literal(value))
     assert got == expected
+
+
+def test_literal_null_type():
+    import unittest.mock as mock
+
+    mock_dtype = mock.Mock()
+    with mock.patch(
+        "bigframes.core.compile.sqlglot.sql.base.sgt.from_bigframes_dtype",
+        return_value="NULL",
+    ):
+        got = sql.to_sql(sql.literal(None, dtype=mock_dtype))
+    assert got == "NULL"