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>
Which is fine so long as everyone knows and remembers that convention... but lenChars is more explicit.