HPCC-12349 JSON outputUnicode outputs extra characters

JSON outputUnicode and outputUtf8 are handling byte count vs character count inconsistently when calling appendJSONStringValue.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
I don't like the name of this function (it's not checking a char is valid, it's only doing half the check (ignoring the first byte).

I'm not sure that pulling it out of utf8CharLen has really made anything any clearer...