buffer: fix indexOf for empty searches

Make searches for empty subsequences do exactly what
`String.prototype.indexOf()` does.

Fixes: https://github.com/nodejs/node/issues/13023
PR-URL: https://github.com/nodejs/node/pull/13024


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>