Add validator to Text type sign-up list field

Adds `NotEmpty` validator and a `StringTrim` filter to make sure the
field value is never the empty string.

module/Activity/src/Form/Element/ValidatedText.php

@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Activity\Form\Element;
+
+use Laminas\Filter\StringTrim;
+use Laminas\Form\Element\Text;
+use Laminas\InputFilter\InputProviderInterface;
+use Laminas\Validator\NotEmpty;
+use Laminas\Validator\ValidatorInterface;
+
+class ValidatedText extends Text implements InputProviderInterface
+{
+    protected ?ValidatorInterface $validator = null;
+
+    /**
+     * Get primary validator.
+     */
+    protected function getValidator(): ValidatorInterface
+    {
+        if (null === $this->validator) {
+            $this->validator = new NotEmpty(NotEmpty::STRING);
+        }
+
+        return $this->validator;
+    }
+
+    /**
+     * Provide default input rules for this element
+     *
+     * Attaches a not empty validator and a string trim filter.
+     *
+     * @inheritDoc
+     */
+    public function getInputSpecification(): array
+    {
+        $spec = [
+            'required'   => true,
+            'filters'    => [
+                ['name' => StringTrim::class],
+            ],
+            'validators' => [
+                $this->getValidator(),
+            ],
+        ];
+
+        $name = $this->getName();
+        if (null !== $name) {
+            $spec['name'] = $name;
+        }
+
+        return $spec;
+    }
+}

module/Activity/src/Form/Signup.php

@@ -4,6 +4,7 @@
 
 namespace Activity\Form;
 
+use Activity\Form\Element\ValidatedText;
 use Activity\Model\SignupField as SignupFieldModel;
 use Activity\Model\SignupList as SignupListModel;
 use Laminas\Captcha\Image as ImageCaptcha;
@@ -135,7 +136,7 @@ protected function createSignupFieldElementArray(SignupFieldModel $field): array
 
         switch ($field->getType()) {
             case 0: //'Text'
-                $result['type'] = 'Text';
+                $result['type'] = ValidatedText::class;
                 break;
             case 1: //'Yes/No'
                 $result['type'] = Radio::class;