service: swap !id.IsNil() for id.Exists()
service: swap !id.IsNil() for id.Exists()

Issue: KBFS-2185

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client