reverted:
--- b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/PrimitiveTypeConstraintValidator.php
+++ a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/PrimitiveTypeConstraintValidator.php
@@ -28,7 +28,7 @@
    */
   public function validate($value, Constraint $constraint) {
 
+    if (!isset($value)) {
-    if (!isset($value) || $value === '') {
       return;
     }