HPCC-9466 Set filter for searching 'unknown' state WUs

An empty string '' is used as a filter when searching 'unknown' state
WUs. The existing filter setting function does not set the filter if
an empty string '' is detected. This fix adds new code to set the filter.
Merge pull request #4627 from wangkx/h9466b

HPCC-9466 Set filter for searching 'unknown' state WUs

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>