Fix viewing some legacy diff notes in discussions

LegacyDiffNotes created by the GitHub importer lack details about the
SHAs they referenced on the GitHub side. This exposes a bug in our
diff viewer selection logic, where we inappropriately pick the image
viewer whenever both old blob and new blob are nil.

In practice, the image diff viewer relies on both old and new blob to
do its work, so this just leads to us being unable to render the diff.

Adding this condition allows us to fall back to the basic text diff,
which is usually sufficient.

app/models/diff_viewer/image.rb

@@ -10,5 +10,13 @@ module DiffViewer
     self.binary = true
     self.switcher_icon = 'doc-image'
     self.switcher_title = _('image diff')
+
+    def self.can_render?(diff_file, verify_binary: true)
+      # When both blobs are missing, we often still have a textual diff that can
+      # be displayed
+      return false if diff_file.old_blob.nil? && diff_file.new_blob.nil?
+
+      super
+    end
   end
 end

changelogs/unreleased/26952-fix-viewing-legacy-diff-notes-in-discussion.yml

+---
+title: Fix viewing GitHub-imported diff notes in discussions
+merge_request: 45920
+author:
+type: fixed

spec/models/diff_viewer/image_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe DiffViewer::Image do
+  describe '.can_render?' do
+    let(:diff_file) { double(Gitlab::Diff::File) }
+    let(:blob) { double(Gitlab::Git::Blob, binary_in_repo?: true, extension: 'png') }
+
+    subject { described_class.can_render?(diff_file, verify_binary: false) }
+
+    it 'returns false if both old and new blob are absent' do
+      allow(diff_file).to receive(:old_blob) { nil }
+      allow(diff_file).to receive(:new_blob) { nil }
+
+      is_expected.to be_falsy
+    end
+
+    it 'returns true if the old blob is present' do
+      allow(diff_file).to receive(:old_blob) { blob }
+      allow(diff_file).to receive(:new_blob) { nil }
+
+      is_expected.to be_truthy
+    end
+
+    it 'returns true if the new blob is present' do
+      allow(diff_file).to receive(:old_blob) { nil }
+      allow(diff_file).to receive(:new_blob) { blob }
+
+      is_expected.to be_truthy
+    end
+
+    it 'returns true if both old and new blobs are present' do
+      allow(diff_file).to receive(:old_blob) { blob }
+      allow(diff_file).to receive(:new_blob) { blob }
+
+      is_expected.to be_truthy
+    end
+  end
+end