Avoid fatal error if field_widget_third_party_settings_form hook returns NULL

Closes #3514262

core/modules/field_ui/src/Form/EntityFormDisplayEditForm.php

@@ -127,13 +127,13 @@ protected function thirdPartySettingsForm(PluginSettingsInterface $plugin, Field
     $this->moduleHandler->invokeAllWith(
       'field_widget_third_party_settings_form',
       function (callable $hook, string $module) use (&$settings_form, $plugin, $field_definition, &$form, $form_state) {
-        $settings_form[$module] = ($settings_form[$module] ?? []) + $hook(
+        $settings_form[$module] = ($settings_form[$module] ?? []) + ($hook(
           $plugin,
           $field_definition,
           $this->entity->getMode(),
           $form,
           $form_state
-        );
+        ) ?? []);
       }
     );
     return $settings_form;