GithubImporter: Fixes merged_by importer when merger user is nil

When the user who merged the Github Pull Request was deleted, github API
sends `nil` in the `merged_by` field. To avoid an exception, when
creating the complementary note that mentions the original merger user,
fallbacks to 'ghost' in the note.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/334213
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65152
Changelog: fixed

lib/gitlab/github_import/importer/pull_request_merged_by_importer.rb

@@ -43,7 +43,7 @@ module Gitlab
 
         def missing_author_note
           s_("GitHubImporter|*Merged by: %{author} at %{timestamp}*") % {
-            author: pull_request.merged_by.login,
+            author: pull_request.merged_by&.login || 'ghost',
             timestamp: pull_request.merged_at
           }
         end

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

@@ -8,13 +8,14 @@ RSpec.describe Gitlab::GithubImport::Importer::PullRequestMergedByImporter, :cle
   let(:project) { merge_request.project }
   let(:merged_at) { Time.new(2017, 1, 1, 12, 00).utc }
   let(:client_double) { double(user: double(id: 999, login: 'merger', email: 'merger@email.com')) }
+  let(:merger_user) { double(id: 999, login: 'merger') }
 
   let(:pull_request) do
     instance_double(
       Gitlab::GithubImport::Representation::PullRequest,
       iid: merge_request.iid,
       merged_at: merged_at,
-      merged_by: double(id: 999, login: 'merger')
+      merged_by: merger_user
     )
   end
 
@@ -48,4 +49,23 @@ RSpec.describe Gitlab::GithubImport::Importer::PullRequestMergedByImporter, :cle
       expect(last_note.author).to eq(project.creator)
     end
   end
+
+  context 'when the merger user is not provided' do
+    let(:merger_user) { nil }
+
+    it 'adds a note referencing the merger user' do
+      expect { subject.execute }
+        .to change(Note, :count).by(1)
+        .and not_change(merge_request, :updated_at)
+
+      metrics = merge_request.metrics.reload
+      expect(metrics.merged_by).to be_nil
+      expect(metrics.merged_at).to eq(merged_at)
+
+      last_note = merge_request.notes.last
+      expect(last_note.note).to eq("*Merged by: ghost at 2017-01-01 12:00:00 UTC*")
+      expect(last_note.created_at).to eq(merged_at)
+      expect(last_note.author).to eq(project.creator)
+    end
+  end
 end