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>
HPCC-12349 Updated based on review

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>