HPCC-9178 Remove several references to UNKNOWN_USER

Code analysis identified several places that reference UNKNOWN_USER which
can be updated with the actual user descriptor. This fix does that

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>
Merge pull request #4265 from RussWhitehead/unkUser

HPCC-9178 Remove several references to UNKNOWN_USER

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <richard.chapman@hpccsystems.com>