FIX: Flair icon being removed when updating other profile info

The commit: 75069ff179

allows users to remove their primary group, but this introduced a bug
where if you were to edit any other profile info like location or
website which is a form on a separate page then the flair dropdown,
would cause the selected flair to be removed.

This fix ensures that if the `primary_group_id` parameter is missing
from the update payload it does not remove the existing
`primary_group_id`. It will only remove the `primary_group_id` if it is
present in the payload and empty.

app/services/user_updater.rb

@@ -90,6 +90,7 @@ class UserUpdater
 
       user.primary_group_id = attributes[:primary_group_id]
     elsif SiteSetting.user_selected_primary_groups &&
+      attributes[:primary_group_id] &&
       attributes[:primary_group_id].blank?
 
       user.primary_group_id = nil

spec/services/user_updater_spec.rb

@@ -273,6 +273,16 @@ describe UserUpdater do
         expect(user.primary_group_id).to eq nil
       end
 
+      it 'does not update when changing other profile data' do
+        SiteSetting.user_selected_primary_groups = true
+        user.groups << new_group
+        user.update(primary_group_id: new_group.id)
+        UserUpdater.new(acting_user, user).update(website: 'http://example.com')
+
+        user.reload
+        expect(user.primary_group_id).to eq new_group.id
+      end
+
       it 'can be removed by the user when setting is enabled' do
         SiteSetting.user_selected_primary_groups = true
         user.groups << new_group