Document names can be blank
When creating a document, there's a check to make sure the document names are not blank. But it is possible to change the name to a blank name when editing the document. This results in a document which can't be accessed anymore. Partially connected to #301 (closed), because the name of the document is blank, the document also can't be accessed over changing the url anymore.
This change should simply disallow editing documents and changing their name to blank names. Additionally it would be nice if there was a migration that made sure that all the currently empty named documents got some sort of filler name.