fs: allow empty string for temp directory prefix

The `fs` lib module's `mkdtemp()` and `mkdtempSync()` methods were
missing a validator, and weren't allowing the empty string as a valid
prefix.

PR-URL: https://github.com/nodejs/node/pull/39028


Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>