Guard against exceptions from unfound DiffNotes

If for some reason or another a DiffNote record is not found, we raise
an ActiveRecord::RecordNotFound error. These don't occur terribly often
(1-2 times/hour) but is worth guarding against.

Changelog: fixed

app/workers/create_note_diff_file_worker.rb

@@ -10,8 +10,8 @@ class CreateNoteDiffFileWorker # rubocop:disable Scalability/IdempotentWorker
   feature_category :code_review
 
   def perform(diff_note_id)
-    diff_note = DiffNote.find(diff_note_id)
+    diff_note = DiffNote.find_by_id(diff_note_id) # rubocop: disable CodeReuse/ActiveRecord
 
-    diff_note.create_diff_file
+    diff_note&.create_diff_file
   end
 end

spec/workers/create_note_diff_file_worker_spec.rb

@@ -14,5 +14,14 @@ RSpec.describe CreateNoteDiffFileWorker do
 
       described_class.new.perform(diff_note.id)
     end
+
+    context "when the supplied diff_note_id doesn't belong to an existing DiffNote" do
+      it "returns nil without raising an error" do
+        expect_any_instance_of(DiffNote).not_to receive(:create_diff_file)
+        .and_call_original
+
+        described_class.new.perform(non_existing_record_id)
+      end
+    end
   end
 end