Suppress warning
ASGARD-1228 - Add callback hook in Glisten for acting upon a newly created WorkflowExecution

Necessary for Asgard to be able to update its in-memory cache immediately after creating a new WorkflowExecution.
Fix unit tests by making callback null safe
Merge pull request #377 from joesondow/ASGARD-1228-Glisten-callback-WorkflowExecution-creation

ASGARD-1228 - Add callback hook in Glisten for acting upon a newly created WorkflowExecution