Issue Summary

Importing cards from Trello where history items are created by deleted users fail with an NPE.

Steps to Reproduce

1. Create a board in Trello and a list using user A
2. Create a card in this board using user B:
   
3. Import it to Jira Cloud using the Trello importer
4. Delete user B from Trello altogether, card will now look like this:
   
5. Import it to Jira Cloud again

Expected Results

Both import attempts work.

Actual Results

First import attempt, while user B still exists, works. Second import fails with an NPE.
This is shown in UI:

Workaround

• Copy the problematic board excluding the activity log and connected users. 
• Import the copied board and it should wokrk