Dispatcher: add unused singular to action plurals (#9734)
Dispatcher: add unused singular to action plurals (#9734)

Tools and humans alike are confused by leaving it empty. It should be inoffensive to resolve the problem this by having an unused singular. Doing anything else would make it so we couldn't have the correct plurals in Slovak, among others.

Follow-up to https://github.com/koreader/koreader/pull/9726
NiLuJe/koreader,poire-z/koreader,NiLuJe/koreader,poire-z/koreader,koreader/koreader,koreader/koreader