AKSubmission: Prevent error if onwer POSTed is empty

When sending POST requests to the owner dispatch views

• submission/owner/edit/
• submission/new/
• submission/owner/

with empty value for owner (only possible with disabled javascript or forged requests), rendering will fail with an internal server error, since URLs for submit_ak and akowner_edit with empty onwer_slug cannot be resolved. There should be a check whether the owner is set before, causing a message and a redirect to submission_overview if the POST value is empty.