HPCC-9255 WsEcl should use IWorkUnit::setXmlParams to set stored values

This will simplify the implementation and solve issues related to empty
vs non-existent elements.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>
Merge branch 'wsecl_setxmlparms9255' into candidate-4.0.0

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>