Use null as the default supported_scopes value.

This fixes the following ticket.

FriendsOfSymfony/FOSOAuthServerBundle#164

#37 tries to default the value as null but since it is initialised as an empty array, it will never be fetched as null by ```getVariable()```. Since FOSOAuthServerBundle implements the scope field as string, doctrine will throw an error because scope is passed in as an array.

Further more, ```checkScope()``` handles string scopes nicely so there is no problem with a string value.