Traceability Report facing "[migration] unknown user" error while exporting it to CSV

Description

Traceability Report facing "[migration] unknown user" error while exporting it to CSV.

Pre-conditions: 

Create a Test Run, keep it as Unassigned in the Server/ Data Center, and migrate this info to a Cloud instance.

How to reproduce:

• Create a Test Execution.
• Add a Test
• Be sure the Test Run created is unassigned in Server/DC
• Migrate this info to a Cloud instance.
• Open the Traceability Report and try to export it via CSV. 

Actual Result:
Facing error  [migration] unknown user when exporting in CSV

Expected Result: 

We should be able to export it to CSV without any error.