getByUserID should return null when no user found
getByUserID should return null when no user found

User::getByUserID() returns an User instance even if the user is not
found.
Let's return nothing if the user has not been loaded...

As a bonus: the query is now a bit faster since the DB engine stops search
at the first record found


Former-commit-id: b3fa0e2c888e8cb51a6ace437efbd53340898037
acliss19xx/concrete5,biplobice/concrete5,concrete5/concrete5,biplobice/concrete5,olsgreen/concrete5,KorvinSzanto/concrete5,triplei/concrete5-8,deek87/concrete5,concrete5/concrete5,haeflimi/concrete5,rikzuiderlicht/concrete5,mlocati/concrete5,MrKarlDilkington/concrete5,jaromirdalecky/concrete5,concrete5/concrete5,deek87/concrete5,deek87/concrete5,hissy/concrete5,MrKarlDilkington/concrete5,rikzuiderlicht/concrete5,concrete5/concrete5,triplei/concrete5-8,KorvinSzanto/concrete5,hissy/concrete5,a3020/concrete5,mlocati/concrete5,jaromirdalecky/concrete5,haeflimi/concrete5,haeflimi/concrete5,acliss19xx/concrete5,mlocati/concrete5,deek87/concrete5,mainio/concrete5,jaromirdalecky/concrete5,mainio/concrete5,hissy/concrete5,triplei/concrete5-8,hissy/concrete5,biplobice/concrete5,haeflimi/concrete5,rikzuiderlicht/concrete5,biplobice/concrete5,jaromirdalecky/concrete5,MrKarlDilkington/concrete5,a3020/concrete5,KorvinSzanto/concrete5,olsgreen/concrete5,mainio/concrete5,acliss19xx/concrete5,mlocati/concrete5,olsgreen/concrete5