Fix failing 500 errors when deploy tokens are used to clone

A DeployToken responds to `:username`, but it returns the username for the
token, not a User object. Don't attempt to log user activity in this case.

Closes gitlab-org/gitlab-ee#7080

app/services/users/activity_service.rb

@@ -11,6 +11,7 @@ module Users
                 author.user
               end
 
+      @user = nil unless @user.is_a?(User)
       @activity = activity
     end
 

spec/services/users/activity_service_spec.rb

@@ -28,6 +28,18 @@ describe Users::ActivityService do
       end
     end
 
+    context 'when a bad object is passed' do
+      let(:fake_object) { double(username: 'hello') }
+
+      it 'does not record activity' do
+        service = described_class.new(fake_object, 'pull')
+
+        expect(service).not_to receive(:record_activity)
+
+        service.execute
+      end
+    end
+
     context 'when last activity is today' do
       let(:last_activity_on) { Date.today }