lib-mail: Change NUL -> 0x80 replacement to use unicode replacement char instead

lib-mail: Change NUL -> 0x80 replacement to use unicode replacement char instead

Using 0x80 produces invalid UTF-8 output, which we should avoid. Some places
in Dovecot code already requires that input is valid UTF-8, so it's also
safer and easier to use the unicode replacement character.

Fixes: Panic: file fts-tokenizer-generic.c: line 210 (fts_tokenizer_generic_simple_next): assertion failed: (char_size > 0)