fs: nullish coalescing to respect zero positional reads

When the file read position is moved passing zero is
not respected and `null` is used instead. PR fixes the
issues by using nullish coalescing which will return
the rhs only when the lhs is `null` or `undefined`;
respecting the zero.

Fixes: https://github.com/nodejs/node/issues/40715

PR-URL: https://github.com/nodejs/node/pull/40716
Fixes: https://github.com/nodejs/node/issues/40699


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>