fix User#random_work to return a value always
fix User#random_work to return a value always

User#random_work currently returns an empty array if completed is empty.
This causes the dashboard view to explode for users with no completed
exercises because asking:

if []

will return true, and continue on to call [].user.username and other
badness. An alternative fix would be to just call [].present? in the
view, but I think this is better in the long term rather than knowing
that random_work might return a single item or an empty collection.

beni55/exercism.io,praveenpuglia/exercism.io,mscoutermarsh/exercism_coveralls,sheekap/exercism.io,exercistas/exercism.io,sheekap/exercism.io,colinrubbert/exercism.io,tejasbubane/exercism.io,colinrubbert/exercism.io,amar47shah/exercism.io,chinaowl/exercism.io,kangkyu/exercism.io,MBGeoff/Exercism.io-mbgeoff,copiousfreetime/exercism.io,Tonkpils/exercism.io,beni55/exercism.io,kizerxl/exercism.io,jtigger/exercism.io,amar47shah/exercism.io,hanumakanthvvn/exercism.io,emilyforst/exercism.io,praveenpuglia/exercism.io,tejasbubane/exercism.io,IanDCarroll/exercism.io,alexclarkofficial/exercism.io,treiff/exercism.io,mhelmetag/exercism.io,mscoutermarsh/exercism_coveralls,kizerxl/exercism.io,IanDCarroll/exercism.io,mscoutermarsh/exercism_coveralls,praveenpuglia/exercism.io,IanDCarroll/exercism.io,emilyforst/exercism.io,mscoutermarsh/exercism_coveralls,amar47shah/exercism.io,colinrubbert/exercism.io,chastell/exercism.io,Tonkpils/exercism.io,mscoutermarsh/exercism_coveralls,kizerxl/exercism.io,alexclarkofficial/exercism.io,sheekap/exercism.io,beni55/exercism.io,chinaowl/exercism.io,jtigger/exercism.io,RaptorRCX/exercism.io,alexclarkofficial/exercism.io,copiousfreetime/exercism.io,mscoutermarsh/exercism_coveralls,MBGeoff/Exercism.io-mbgeoff,mscoutermarsh/exercism_coveralls,treiff/exercism.io,k4rtik/exercism.io,tejasbubane/exercism.io,chinaowl/exercism.io,hanumakanthvvn/exercism.io,RaptorRCX/exercism.io,treiff/exercism.io,bmulvihill/exercism.io,RaptorRCX/exercism.io,jtigger/exercism.io,tejasbubane/exercism.io,emilyforst/exercism.io,praveenpuglia/exercism.io,nathanbwright/exercism.io,MBGeoff/Exercism.io-mbgeoff,hanumakanthvvn/exercism.io,RaptorRCX/exercism.io,copiousfreetime/exercism.io,nathanbwright/exercism.io,sheekap/exercism.io,IanDCarroll/exercism.io,mscoutermarsh/exercism_coveralls,exercistas/exercism.io,treiff/exercism.io,mhelmetag/exercism.io,Tonkpils/exercism.io,bmulvihill/exercism.io,mhelmetag/exercism.io,exercistas/exercism.io,jtigger/exercism.io,kangkyu/exercism.io,bmulvihill/exercism.io,chastell/exercism.io,copiousfreetime/exercism.io,mscoutermarsh/exercism_coveralls,nathanbwright/exercism.io,bmulvihill/exercism.io,mscoutermarsh/exercism_coveralls,k4rtik/exercism.io,nathanbwright/exercism.io,k4rtik/exercism.io,Tonkpils/exercism.io,chastell/exercism.io,kangkyu/exercism.io,hanumakanthvvn/exercism.io,mhelmetag/exercism.io