For the moment it's not possible to put an empty string back into a Dictionary item that once had a value.
Is there a reason for this?

While developing a website I regulary put some dummy text in the dictionary items (so you get that green check) but I'm not able to put in an empty string again (so you get a red sign again).

The reason why I wan't to do this is double. First of all I want to show this list of dictionary items to our translator so he/she can translate all the items with a red sign. Second one is that I sometimes want an empty sting in an items, for instance when one language need some special text and others dont.