refactor: fix ops.MapOp

Author: chelsea-lin

bigframes/core/compile/sqlglot/expressions/generic_ops.py

@@ -107,6 +107,7 @@ def _(expr: TypedExpr, op: ops.MapOp) -> sge.Expression:
             sge.If(this=sge.convert(key), true=sge.convert(value))
             for key, value in op.mappings
         ],
+        default=expr.expr,
     )
 
 

tests/unit/core/compile/sqlglot/expressions/snapshots/test_generic_ops/test_map/out.sql

@@ -5,7 +5,7 @@ WITH `bfcte_0` AS (
 ), `bfcte_1` AS (
   SELECT
     *,
-    CASE `string_col` WHEN 'value1' THEN 'mapped1' END AS `bfcol_1`
+    CASE `string_col` WHEN 'value1' THEN 'mapped1' ELSE `string_col` END AS `bfcol_1`
   FROM `bfcte_0`
 )
 SELECT