Workflow Designer throws NPE if imported workflow links to missing statuses

1. Import workflow XML where some steps link to status ID-s that don't exist in this instance of JIRA
2. Open that workflow in diagram mode
3. BUG: workflow doesn't open, an "unexpected error" message is shown. The logs contain the stacktrace of a NullPointerException.

The problem is that currently WFD code assumes that all steps have linked statuses and that such statuses exist. It is not necessarily the case for workflows imported as XML.