multi-user picker custom field which is made "required/mandatory" in field configuration,allows white space and comma.

Issue Summary

While creating an issue, multi-user picker custom field which is made "required/mandatory" in field configuration,allows white space and comma.
The expected behavior is it should allow creating an issue. 

Steps to Reproduce

1. Create a "user picker (multiple users)" custom filed.
2. Add it to any of the project say "aaa" in create,view,edit screen. 
3. Create an issue in "aaa"  project and in the multiuser picker field type space or comma. Attachment for reference.
4. Hit create and the issue creates successfully.

Expected Results

• Jira should not allow creating the issue, throwing entered value is invalid or username not found.

Actual Results

• Issue getting created without any error despite mandatory field is empty.

Workaround

None

Currently there is no known workaround for this behavior. A workaround will be added here when available