Return explicit `nil` for all awardables with no todos

This cosmetic change makes it explicit that we create todos only
for a couple of awardables by adding an `else` branch returning `nil`

app/services/award_emojis/add_service.rb

@@ -37,7 +37,8 @@ module AwardEmojis
           awardable.noteable unless awardable.for_personal_snippet?
         when MergeRequest, Issue
           awardable
-        when Snippet
+        else
+          # No todos for all other awardables
           nil
         end
       end