HPCC-12349 Fix toJSON output of extra characters for unicode strings

JSON outputUnicode and outputUtf8 were handling byte count vs character count inconsistently when calling appendJSONStringValue.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
I'll keep this idea in mind, but don't think I'll make the change for this issue.  The main thing that is lost I think would be graceful handling / escaping of invalid utf8 formatted strings... might be tricky to reconcile character counts with bad formatting.