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

Due to an oversight in de3600452b, 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/backend/parser/gram.y

@@ -14265,7 +14265,7 @@ json_table_column_definition:
 				}
 			| ColId Typename
 				EXISTS json_table_column_path_clause_opt
-				json_behavior_clause_opt
+				json_on_error_clause_opt
 				{
 					JsonTableColumn *n = makeNode(JsonTableColumn);
 
@@ -14276,8 +14276,8 @@ json_table_column_definition:
 					n->wrapper = JSW_NONE;
 					n->quotes = JS_QUOTES_UNSPEC;
 					n->pathspec = (JsonTablePathSpec *) $4;
-					n->on_empty = (JsonBehavior *) linitial($5);
+					n->on_empty = NULL;
-					n->on_error = (JsonBehavior *) lsecond($5);
+					n->on_error = (JsonBehavior *) $5;
 					n->location = @1;
 					$$ = (Node *) n;
 				}

src/test/regress/expected/sqljson_jsontable.out

@@ -1067,3 +1067,8 @@ CREATE OR REPLACE VIEW public.jsonb_table_view7 AS
         ) sub
 DROP VIEW jsonb_table_view7;
 DROP TABLE s;
+-- Prevent ON EMPTY specification on EXISTS columns
+SELECT * FROM JSON_TABLE(jsonb '1', '$' COLUMNS (a int exists empty object on empty));
+ERROR:  syntax error at or near "empty"
+LINE 1: ...sonb '1', '$' COLUMNS (a int exists empty object on empty));
+                                                               ^

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));