Issue #3472012 - Fix warnings

Related to #3472012

tests/src/Unit/Validators/AlphaValidatorTest.php

@@ -23,7 +23,7 @@ class AlphaValidatorTest extends BaseValidator {
   /**
    * {@inheritdoc}
    */
-  public function setUp() {
+  public function setUp(): void {
     parent::setUp();
 
     $this->plugin = new AlphaValidator();