(re)fix Task#update_tasks_counts where assigned is nil
(re)fix Task#update_tasks_counts where assigned is nil

This was previously fixed in a19907fb92aeb2380239e7c03541dffddd9eac61
but the code was reverted in 07e56a3e327af2e84b9d7da2ceb2b63ff7c1fe82

vveliev/teambox,ccarruitero/teambox,crewmate/crewmate,vveliev/teambox,vveliev/teambox,vveliev/teambox,ccarruitero/teambox,teambox/teambox,irfanah/teambox,nfredrik/teambox,rahouldatta/teambox,codeforeurope/samenspel,teambox/teambox,codeforeurope/samenspel,crewmate/crewmate,irfanah/teambox,nfredrik/teambox,crewmate/crewmate,rahouldatta/teambox,irfanah/teambox,rahouldatta/teambox,irfanah/teambox,nfredrik/teambox,teambox/teambox,nfredrik/teambox