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

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e3c1393efc31ac70de7b68e9a283ec3f2d7f9bd2

Modified Files
--------------
src/backend/parser/gram.y                       | 6 +++---
src/test/regress/expected/sqljson_jsontable.out | 5 +++++
src/test/regress/sql/sqljson_jsontable.sql      | 3 +++
3 files changed, 11 insertions(+), 3 deletions(-)