Fix truncated_diff_lines for legacy notes without a line

It appears that some legacy diff notes on GitLab.com have invalid line codes,
which means truncated_diff_lines won't work. This is just a band-aid solution,
as neither the diff nor the discussion will display correctly for these notes,
but they won't cause a 500 error.

app/models/concerns/discussion_on_diff.rb

@@ -37,6 +37,8 @@ module DiscussionOnDiff
 
   # Returns an array of at most 16 highlighted lines above a diff note
   def truncated_diff_lines(highlight: true)
+    return [] if diff_line.nil? && first_note.is_a?(LegacyDiffNote)
+
     lines = highlight ? highlighted_diff_lines : diff_lines
 
     initial_line_index = [diff_line.index - NUMBER_OF_TRUNCATED_DIFF_LINES + 1, 0].max

changelogs/unreleased/42160-error-500-loading-merge-request-undefined-method-index-for-nil-nilclass.yml

+---
+title: Fix 500 error when loading a merge request with an invalid comment
+merge_request: 16795
+author:
+type: fixed

spec/models/concerns/discussion_on_diff_spec.rb

@@ -20,6 +20,16 @@ describe DiscussionOnDiff do
         expect(truncated_lines).not_to include(be_meta)
       end
     end
+
+    context "when the diff line does not exist on a legacy diff note" do
+      it "returns an empty array" do
+        legacy_note = LegacyDiffNote.new
+
+        allow(subject).to receive(:first_note).and_return(legacy_note)
+
+        expect(truncated_lines).to eq([])
+      end
+    end
   end
 
   describe '#line_code_in_diffs' do