Gracefully handle orphaned write deploy keys in /internal/post_receive

On GitLab.com, there are write deploy keys with no associated users.
Pushes with these deploy keys end with an Error 500 since we attempt
to look up redirect message. If there is no user, don't attempt
to display a redirect message.

Closes #41466

changelogs/unreleased/sh-handle-orphaned-deploy-keys.yml

+---
+title: Gracefully handle orphaned write deploy keys in /internal/post_receive
+merge_request:
+author:
+type: fixed

lib/api/internal.rb

@@ -190,9 +190,12 @@ module API
 
         project = Gitlab::GlRepository.parse(params[:gl_repository]).first
         user = identify(params[:identifier])
+
+        # A user is not guaranteed to be returned; an orphaned write deploy
+        # key could be used
+        if user
           redirect_message = Gitlab::Checks::ProjectMoved.fetch_redirect_message(user.id, project.id)
-        if redirect_message
-          output[:redirected_message] = redirect_message
+          output[:redirected_message] = redirect_message if redirect_message
         end
 
         output

spec/requests/api/internal_spec.rb

@@ -784,6 +784,16 @@ describe API::Internal do
         expect(json_response["redirected_message"]).to eq(project_moved.redirect_message)
       end
     end
+
+    context 'with an orphaned write deploy key' do
+      it 'does not try to notify that project moved' do
+        allow_any_instance_of(Gitlab::Identifier).to receive(:identify).and_return(nil)
+
+        post api("/internal/post_receive"), valid_params
+
+        expect(response).to have_gitlab_http_status(200)
+      end
+    end
   end
 
   describe 'POST /internal/pre_receive' do