Add missing initialization in transformJsonFuncExpr()

de3600452b added some code for the new JSON_TABLE_OP to that function
but missed to initialize the default_format variable.

Reported-by: Erik Rijkers <er@xs4all.nl>
Discussion: https://postgr.es/m/254b2fa2-2f6b-a30a-20ee-21f8a2c12a50@xs4all.nl

src/backend/parser/parse_expr.c

@@ -4272,6 +4272,7 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 			break;
 		case JSON_TABLE_OP:
 			func_name = "JSON_TABLE";
+			default_format = JS_FORMAT_JSONB;
 			break;
 		default:
 			elog(ERROR, "invalid JsonFuncExpr op %d", (int) func->op);