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>
It is possible just to check the first byte.  And we have two versions of utf8CharLen, unfortunately this version validates the character format.  I'll change this.