Fix login issue when choosing default directory.
Fix login issue when choosing default directory.

The config file was being written with empty data.

Fixes exercism/exercism.io#998

zabawaba99/cli,lcowell/cli,Tonkpils/cli,zabawaba99/cli,yeah-right/cli,arvidsaur/cli,neslom/cli,yeah-right/cli,lcowell/cli,yeah-right/cli,Tonkpils/cli,Tonkpils/cli,zabawaba99/cli,neslom/cli,exercism/cli,exercism/cli,arvidsaur/cli,lcowell/cli,arvidsaur/cli,neslom/cli