fs: ensure options.flag defaults to 'r' in readFile

When passing {} or { encoding: 'utf8' } as options to readFile, the
flag is not defaulted to 'r' unlike normal fs. This fix makes
fs.promises.readFile() act consistent with fs.readFile().

It also fixes another issue with fs.promises.readfile() where it
returned a Buffer instead of an empty string when encoding is provided.

PR-URL: https://github.com/nodejs/node/pull/20268


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>