Fix Persona reloaded
Fix Persona reloaded

When user is disconnected, current_user_mail must be null and not an empty
string
See #418

FreshRSS/FreshRSS,marienfressinaud/FreshRSS,aledeg/FreshRSS,Dolomich/FreshRSS,romibi/FreshRSS,FreshRSS/FreshRSS,superboum/FreshRSS,aledeg/FreshRSS,samuelstein/FreshRSS,romibi/FreshRSS,Alkarex/FreshRSS,marienfressinaud/FreshRSS,FreshRSS/FreshRSS,romibi/FreshRSS,asmundg/FreshRSS,Alkarex/FreshRSS,marienfressinaud/FreshRSS,asmundg/FreshRSS,aledeg/FreshRSS,Dolomich/FreshRSS,superboum/FreshRSS,Alkarex/FreshRSS,aledeg/FreshRSS,romibi/FreshRSS,plopoyop/FreshRSS,samuelstein/FreshRSS,asmundg/FreshRSS,FreshRSS/FreshRSS,samuelstein/FreshRSS,Alkarex/FreshRSS,plopoyop/FreshRSS,superboum/FreshRSS,Dolomich/FreshRSS,marienfressinaud/FreshRSS