Disallow specifying ON_ERROR option without value.

The ON_ERROR option of the COPY command previously allowed omitting
its value, which was inconsistent with the syntax synopsis in the
documentation and the behavior of other non-boolean COPY options.

This change enforces providing a value for the ON_ERROR option,
ensuring consistency across other non-boolean options and aligning
with the documented syntax.

Author: Atsushi Torikoshi
Reviewed-by: Masahiko Sawada
Discussion: https://postgr.es/m/a9770bf57646d90dedc3d54cf32634b2%40oss.nttdata.com

Author: MasahikoSawada

src/backend/commands/copy.c

@@ -392,24 +392,17 @@ defGetCopyHeaderChoice(DefElem *def, bool is_from)
 static CopyOnErrorChoice
 defGetCopyOnErrorChoice(DefElem *def, ParseState *pstate, bool is_from)
 {
-	char	   *sval;
+	char	   *sval = defGetString(def);
 
 	if (!is_from)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("COPY ON_ERROR cannot be used with COPY TO"),
 				 parser_errposition(pstate, def->location)));
 
-	/*
-	 * If no parameter value given, assume the default value.
-	 */
-	if (def->arg == NULL)
-		return COPY_ON_ERROR_STOP;
-
 	/*
 	 * Allow "stop", or "ignore" values.
 	 */
-	sval = defGetString(def);
 	if (pg_strcasecmp(sval, "stop") == 0)
 		return COPY_ON_ERROR_STOP;
 	if (pg_strcasecmp(sval, "ignore") == 0)