Gracefully handle deleted user in audit event

Closes gitlab-org/gitlab-ce#14425

app/services/audit_event_service.rb

@@ -8,7 +8,7 @@ class AuditEventService
     old_access_level = @details[:old_access_level]
     author_name = @author.name
     user_id = member.id
-    user_name = member.user.name
+    user_name = member.user ? member.user.name : 'Deleted User'
 
     @details =
       case action

spec/services/audit_event_service_spec.rb

+require 'spec_helper'
+
+describe AuditEventService, services: true do
+  let(:project) { create(:project) }
+  let(:user) { create(:user) }
+  let(:project_member) { create(:project_member, user: user) }
+  let(:service) { described_class.new(user, project, { action: :destroy }) }
+
+  describe '#for_member' do
+    it 'generates event' do
+      event = service.for_member(project_member).security_event
+      expect(event[:details][:target_details]).to eq(user.name)
+    end
+
+    it 'handles deleted users' do
+      expect(project_member).to receive(:user).and_return(nil)
+
+      event = service.for_member(project_member).security_event
+      expect(event[:details][:target_details]).to eq('Deleted User')
+    end
+  end
+end