readline: refactor filter() callback

Use construct that always returns a boolean for `filter()` callback.

PR-URL: https://github.com/nodejs/node/pull/17858


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>