Allow to continue on empty steps

src/EventListener/PrepareFomDataListener.php

@@ -38,7 +38,14 @@ public function __invoke(array &$submitted, array &$labels, array $fields, Form
         $labelsBag = new ParameterBag($labels);
 
         $pageSwitchValue = $submittedBag->get('mp_form_pageswitch', '');
-        $submittedBag->remove('mp_form_pageswitch');
+
+        // Remove the page switch value field from the submitted data if that's the last step, so it's not passed on
+        // in e-mail notifications and the like. However, in intermediate steps we need it because otherwise it's not
+        // possible to have an empty step (e.g. only explanation fields) as the step data would be empty and
+        // getFirstInvalidStep() would return the one before the empty step.
+        if ($manager->isLastStep()) {
+            $submittedBag->remove('mp_form_pageswitch');
+        }
 
         // Store data in session
         $stepData = $manager->getDataOfCurrentStep();