The Default reporter can not be removed from E-mail handlers

Issue Summary

Once you add a default reporter to an e-mail handler, that default reporter can not be removed from it.

Steps to Reproduce

1. Create a new e-mail handler (tested with "Add comment to the entire email body" and "Create a new issue or add a comment to an existing issue")
2. Add a Default Reporter to the handler and save it
3. Click to edit the e-mail handler and try to remove the Default Reporter by erasing the field value

Expected Results

The Default Reporter should be removed

Actual Results

Although you are able to erase the field and save it, the default reporter is still there after you click to save.

Workaround

Delete the handler and create a new one.