Blank kevalues don't get purged resulting in issues #158

Description of feature

This happens mostly when mass editing entities. For example when selecting all monsters and turning them into squadmakers, select the "model" keyvalue and turn it into "new_model". This results in entities that didnt have the model now do but its value is blank.
Ingame, for some reason, a blank value in model makes the npc completely invisible (and of course not working).
Other worse issues might happen if the blank values are in other more important keyvalues. I have not tested.
Previously, I think, anything with blank keyvalues was purged when closing or even when saving since its the first time I've encountered this problem and I've been mass editing since previous versions.

I've labeled this as a feature request instead of a bug, but it could be considered both. Since its just annoying but easily fixable with some notepad editing I didnt think of it as a dangerous bug that should be a priority.