FIX: guardian always got user but sometimes it is anonymous (#9342)

* FIX: guardian always got user but sometimes it is anonymous

```
  def initialize(user = nil, request = nil)
    @user = user.presence || AnonymousUser.new
    @request = request
  end
```

AnonymouseUser defines `blank?` method
```
  class AnonymousUser
    def blank?
      true
    end
    ...
  end
```
so if we would use @user.present? it would be correct, however, just @user is always true

lib/guardian.rb

@@ -319,7 +319,7 @@ class Guardian
   # Support sites that have to approve users
   def can_access_forum?
     return true unless SiteSetting.must_approve_users?
-    return false unless @user
+    return false if anonymous?
 
     # Staff can't lock themselves out of a site
     return true if is_staff?
@@ -442,7 +442,7 @@ class Guardian
   end
 
   def can_export_entity?(entity)
-    return false unless @user
+    return false if anonymous?
     return true if is_admin?
     return entity != 'user_list' if is_moderator?