Wikimedia: fix .+prenom issue when no argument is given

Summary: Handle the case .+prenom is used without argument.

Test Plan:
Tested on Dæghrefn:

16:10:49 <Dereckson> .+prenom
16:10:49 <Daeghrefn> Quel prénom ajouter ? e.g. .+prenom Aude F

Reviewers: Ash-Crow, dereckson

Differential Revision: http://devcentral.nasqueron.org/D410