Traceability report fails to generate if there are test runs executed by "[migration] unknown user"

The traceability report fails to generate if it includes test runs which were executed by "[migration] unknown user".

How to reproduce

1. Go to the traceability report page in Apps > Xray > Traceability;
2. Make sure there are test runs associated with a coverable issue (for example a story), where the executed by field is “[migration] unknown user”;
3. Apply any filter in the traceability report page that returns that coverable issue as well as the associated test runs;

Actual Result

The report fails to generate and an error is displayed.

Expected Result

The report is correctly generated.