BURs: fix exception in category change requests for new tags.

`category blah -> character` failed when `blah` didn't exist yet.

test/unit/bulk_update_request_test.rb

@@ -160,6 +160,14 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
         assert_equal(Tag.categories.meta, tag.reload.category)
       end
+
+      should "work for a new tag" do
+        bur = FactoryBot.create(:bulk_update_request, :script => "category new_tag -> meta")
+        bur.approve!(@admin)
+
+        assert_not_nil(Tag.find_by_name("new_tag"))
+        assert_equal(Tag.categories.meta, Tag.find_by_name("new_tag").category)
+      end
     end
 
     context "with an associated forum topic" do