SQL/JSON: Prevent ON EMPTY for EXISTS columns in JSON_TABLE()

Due to an oversight in de3600452b61, the ON EMPTY clause was
incorrectly allowed in the EXISTS column. Fix the grammar to prevent
this.

Discussion: https://postgr.es/m/CA%2BHiwqHh3YDXTpccgAo4CdfV9Mhy%2Bmg%3Doh6t8rfM5uLW1BJN4g%40mail.gmail.com

src/test/regress/sql/sqljson_jsontable.sql

@@ -518,3 +518,6 @@ SELECT sub.* FROM s,
 \sv jsonb_table_view7
 DROP VIEW jsonb_table_view7;
 DROP TABLE s;
+
+-- Prevent ON EMPTY specification on EXISTS columns
+SELECT * FROM JSON_TABLE(jsonb '1', '$' COLUMNS (a int exists empty object on empty));