Native engine crashes on concat_ws with literal NULL separator #3339

Description

When Spark's ConstantFolding optimizer rule is disabled, concat_ws with a literal NULL separator reaches the native engine, which crashes with:

Expected string literal, got None.

How to Reproduce

SET spark.sql.optimizer.excludedRules=org.apache.spark.sql.catalyst.optimizer.ConstantFolding;
SELECT concat_ws(NULL, 'a', 'b');

Expected Behavior

The native engine should handle a NULL separator gracefully — either returning NULL (matching Spark behavior) or falling back to Spark.