Ensure broken diff notes don't error out

This is to prevent many issues when rendering notes that are
corrupted in some way. There seem to be multiple bugs that
allow this to happen, and the result is that it fails to
render the entire block of notes.

app/models/concerns/discussion_on_diff.rb

@@ -40,7 +40,7 @@ module DiscussionOnDiff
   # Returns an array of at most 16 highlighted lines above a diff note
   def truncated_diff_lines(highlight: true, diff_limit: nil)
     return [] unless on_text?
-    return [] if diff_line.nil? && first_note.is_a?(LegacyDiffNote)
+    return [] if diff_line.nil?
 
     diff_limit = [diff_limit, NUMBER_OF_TRUNCATED_DIFF_LINES].compact.min
     lines = highlight ? highlighted_diff_lines : diff_lines

changelogs/unreleased/28376-merge-request-comments-have-stopped-appearing-for-the-whole-project.yml

+---
+title: Fix Merge Request comments when some notes are corrupt
+merge_request: 23786
+author:
+type: fixed

spec/models/concerns/discussion_on_diff_spec.rb

@@ -59,6 +59,18 @@ describe DiscussionOnDiff do
       end
     end
 
+    context "when the diff line does not exist on a corrupt diff note" do
+      subject { create(:diff_note_on_merge_request, line_number: 18).to_discussion }
+
+      before do
+        allow(subject).to receive(:diff_line) { nil }
+      end
+
+      it "returns an empty array" do
+        expect(truncated_lines).to eq([])
+      end
+    end
+
     context 'when the discussion is on an image' do
       subject { create(:image_diff_note_on_merge_request).to_discussion }