Show [deleted sentence] instead of empty text for replacements w/ empty requests

Actually it hasn't been possible yet to create requests with empty content so this behavior will introduced with this MR. The replacement of empty content with the [deleted sentence] is handled by the frontend, I thought this would be the nicest and cleanest option. We could also think about writing [deleted sentence] into the database (as some kind of null value), would have a slight advantage when it comes to agreement requests of requests with empty content (since you would have [deleted sentence] as default text). Don't have any strong feelings about it so I decided to keep it that way for now.

Closes #64 (closed)