Updating sprint with a CSV through external system import doesn't clear sprints

Issue Summary

When updating sprints by uploading a CSV through external system import, the importer overwrites any sprints the issue had, but it doesn't remove the sprint from an issue when the column is empty.

Steps to Reproduce

1. On a project, look for a few issues already assigned to sprints
2. Create a CSV with the issue keys, summaries, and sprint
3. Update the sprints from the issues by adding different sprint numbers on the CSV and in one of the examples, leave the sprint empty
   Example:

   Key	Summary	Sprint	Sprint
   TEST-1	test 1	1
   TEST-2	test 2
   TEST-3	test 3	1	2

Expected Results

All issues have the sprints updated according to the CSV, meaning the issue on the CSV that has no sprint numbers has all the sprints removed.

Using the example:

• TEST-1 would have all its sprints overwritten to sprint 1
• TEST-2 wouldn't be associated with any sprints
• TEST-3 would be associated with sprints 1 and 2

Actual Results

The sprints are only updated for the issues that have a sprint associated (using null or an invalid sprint number will only throw an error and still doesn't update the sprint).
The issue with no sprints on the CSV keeps all the old sprints.

Using the example:

• TEST-1 would have all its sprints overwritten to sprint 1
• TEST-2 keeps all the existing sprints
• TEST-3 would be associated with sprints 1 and 2

Workaround

• Create a new sprint through the UI, to be deleted at the end of the process
• Add the sprint number from this created sprint on the CSV
• Use the external system import to update the issues
• Delete the sprint created through the UI