HPCC-11404 Fix regression in Utf8 output

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5853 from afishbeck/utf8sizeregression

HPCC-11404 Fix regression in Utf8 output

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>