Don't stop converting subsequent chunks just because the length of one of them
was 0: this can happen if the first character of a string is a NUL or if there
are two (or more) NULs in it later.
Simply remove the check for this and continue as usual even in this case.
Also add a unit test verifying that we do translate NULs in input into NULs in
output.
Closes#16620.
(this is a backport of f99ff49 from master)
264f01824b