Ensure MR diffs always exist in the PR importer

In rare cases it could happen that an MR was created, but creating the
MR diffs somehow failed (e.g. due to an error). This commit adds an
additional check to make sure MR diffs are always present when importing
GitHub pull requests.

changelogs/unreleased/pr-importer-io-extra-error-handling.yml

+---
+title: Ensure MR diffs always exist in the PR importer
+merge_request:
+author:
+type: fixed

lib/gitlab/github_import/importer/pull_request_importer.rb

@@ -104,7 +104,8 @@ module Gitlab
           # first save the diff, then populate it.
           diff =
             if already_exists
-              merge_request.merge_request_diffs.take
+              merge_request.merge_request_diffs.take ||
+                merge_request.merge_request_diffs.build
             else
               merge_request.merge_request_diffs.build
             end

spec/lib/gitlab/github_import/importer/pull_request_importer_spec.rb

@@ -276,5 +276,17 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
       expect(diff.merge_request_diff_commits.exists?).to eq(true)
     end
+
+    context 'when the merge request exists' do
+      it 'creates the merge request diffs if they do not yet exist' do
+        mr, _ = importer.create_merge_request
+
+        mr.merge_request_diffs.delete_all
+
+        importer.insert_git_data(mr, true)
+
+        expect(mr.merge_request_diffs.exists?).to eq(true)
+      end
+    end
   end
 end