WorkflowPuger does not purge terminated Workflow

**Describe the bug**
I am using WorkflowPurger to cleanup old workflow persisted entries. Works fine as long as the have a CompletedTime. This seems to be not the case for WF with Status Terminated (3). I am using EF 5 and the default provider for it.

**To Reproduce**
Steps to reproduce the behavior:
Terminate Workflow with Exception and have .OnError(Terminate) in the Workflow definition.

**Expected behavior**
I should be possible to purge a terminated Workflow with CompletedTime == null

**Additional context**
Simple Suggestion: Allow a nullable datetime in Method PurgeWorkflows.