src: fix TextDecoder final flush size calculation

Flushing a TextDecoder with a zero-sized input and pending
incomplete characters was failing when fatal: false.

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/39737


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>