Project migration fails when issues have null priority

Issue Summary

When issues have null priorities, the project migration fails.

Steps to Reproduce

One way to reproduce this problem is to create a new issue and remove the priority directly from the Jira database. There might be other ways to run into this issue though.

Expected Results

Migration succeeds by assigning a default priority value to issues with missing priority.

Actual Results

The project migration completes with a FAILED status.

Workaround

Option 1

1. Run this JQL

   priority is null

1. This will show all issues with missing priority
2. Bulk update the issues to have a priority value.

Option 2

1. Go to the Jira database and assign priorities to all issues with null priorities.
2. Sample SQL to make the change (might not work for all database types):

   update jiraissue set priority=2 where id in (select id from jiraissue where priority is null);

   (make sure to update the priority value accordingly)

Option 3

1. If the project has a priority scheme with default priority enabled. direction as given below.
2.  Then, enable this feature flag (com.atlassian.jira.migration.assign.default.issue.priority) so that JCMA will set the default priority whenever an issue found with null or empty priority.  Please refer to this document https://confluence.atlassian.com/jirakb/enable-dark-feature-in-jira-959286331.html to add feature flags.