Service desk mail rejection on empty subject

Summary

When a user send an email to a service desk enabled email address, but leaves the subject completely empty, the mail is rejected.

Steps to reproduce

Send an email with empty subject to a service desk enabled email address, and see the rejection the come in.

What is the current bug behavior?

The user gets the following automated reply by email:

Unfortunately, your email message to GitLab could not be processed.

Gitlab::Email::InvalidIssueError

This mail has the subject "[Rejected]".

What is the expected correct behavior?

I would expect that the ticket is still opened, but it uses a placeholder subject such as "(no subject)", similar to how Outlook and Roundcube handle such subjects.

Possible fixes

If the subject of the incoming mail is empty, fallback to a default.