Don't set a default value if the field already has a value

Closes #3432131

src/WebformSubmissionHandler.php

@@ -120,6 +120,10 @@ class WebformSubmissionHandler {
       if (!empty($values) && empty($values['is_error'])) {
         unset($values['is_error']);
         foreach ($values as $field => $value) {
+          $existingValue = $webform_submission->getElementData($field);
+          if ($existingValue !== NULL && $existingValue !== '') {
+            continue;
+          }
           $this->setDefaultValue($form, $form_state, $webform_submission, $field, $value);
         }
       }