diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItemBase.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItemBase.php
index 4772983..11ccec6 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItemBase.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItemBase.php
@@ -40,4 +40,11 @@ public static function propertyDefinitions(FieldStorageDefinitionInterface $fiel
     return $properties;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function isEmpty() {
+    return $this->value === NULL || $this->value === '';
+  }
+
 }
diff --git a/core/modules/user/src/UserStorageSchema.php b/core/modules/user/src/UserStorageSchema.php
index 447469d..fa1d491 100644
--- a/core/modules/user/src/UserStorageSchema.php
+++ b/core/modules/user/src/UserStorageSchema.php
@@ -51,7 +51,7 @@ protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $st
         case 'name':
           // Improves the performance of the user__name index defined
           // in getEntitySchema().
-          $schema['fields'][$field_name]['not null'] = TRUE;
+//          $schema['fields'][$field_name]['not null'] = TRUE;
           break;
 
         case 'mail':