Orphaned entities cause Jira Site import to fail

Issue Summary

If a Jira site export contains corrupted data it causes the complete site import to fail.

This problem manifests itself under various circumstances: (see internal notes in the first comment)

Steps to Reproduce

1. Generate an export file from a jira instance (server or cloud)
2. edit the xml export file to create one piece of data to be "orphaned" (for example, remove a <project
   entity, but leave the <issues
   )

Expected Results

The import complete successfully. The orphaned entities are ignored.

Actual Results

The import fails with an unhelpful error message such as

Please contact Atlassian support. org.postgresql.util.PSQLException: ERROR: column "issue" contains null values"

Workaround

Manually edit the XML export file to remove the orphaned entities.