HPCC-10749 Search WU/Query with non-existence of property/attr

Using the existing code, WUQuery?State=unknown does not return "unknown"
WUs. The "unknown" WUs are WUs which do not have the @state or @state =
"". We do not suport any xpath syntax that allow you to test for the non
-existence of a property or attribute. In this fix, the code is added to
filter ECL WUs (and DFU WU, Query) with non-existence of a property or
attribute.
HPCC-10749 Revise based on code review

1. Clone 'unknownAttributes'; 2. Use sortElements to avoid duplicated
code; 3. Move 'cSort sort' into block; 4. Use simple array lookup.
HPCC-10749 Rebase and remove extra checks, etc.
Merge pull request #5343 from wangkx/h10749

HPCC-10749 Search WU/Query with non-existence of property/attr

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>