[Fizz Node] Fix null bytes written at text chunk boundaries (#26228)

We encode strings 2048 UTF-8 bytes at a time. If the string we are
encoding crosses to the next chunk but the current chunk doesn't fit an
integral number of characters, we need to make sure not to send the
whole buffer, only the bytes that are actually meaningful.

Fixes #24985. I was able to verify that this fixes the repro shared in
the issue (be careful when testing because the null bytes do not show
when printed to my terminal, at least). However, I don't see a clear way
to add a test for this that will be resilient to small changes in how we
encode the markup (since it depends on where specific multibyte
characters fall against the 2048-byte boundaries).