Fix custom emoji category creation silently erroring out on duplicate category (#12647)

Instead, just re-use the existing category if any.

Fixes #12608

app/models/form/custom_emoji_batch.rb

@@ -40,7 +40,7 @@ class Form::CustomEmojiBatch
       if category_id.present?
         CustomEmojiCategory.find(category_id)
       elsif category_name.present?
-        CustomEmojiCategory.create!(name: category_name)
+        CustomEmojiCategory.find_or_create_by!(name: category_name)
       end
     end