HoldingPen: cleared urls cause error state

[fschwenn]

If one removes the urls of a user submission and accepts it, it ends up in error state. If one opens it again, there is an empty urls-field. If one removes that empty field, the submission is digested properly.

Expected Behavior

Removing the urls should really remove the urls.

Current Behavior

As it seems removing the urls in fact replaces them with an empty url (which is not valid)

Steps to Reproduce (for bugs)

Make a user submission with url. Remove the url. Accept the submission in the holdingpen.
It will end up in error state.