ensure nulls_first is provided

Author: maxcountryman

tests/sqlparser_common.rs

@@ -939,13 +939,15 @@ fn parse_listagg() {
                 quote_style: None,
             }),
             asc: None,
+            nulls_first: None,
         },
         OrderByExpr {
             expr: Expr::Identifier(Ident {
                 value: "username".to_string(),
                 quote_style: None,
             }),
             asc: None,
+            nulls_first: None
         },
     ];
     assert_eq!(