Add new constraint that allows blank values during install

Closes #3461428

core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/AfterInstallConstraint.php

+<?php
+
+namespace Drupal\Core\Validation\Plugin\Validation\Constraint;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Validation\Attribute\Constraint;
+use Symfony\Component\Validator\Constraints\Sequentially;
+
+/**
+ * AfterInstallConstraint constraint.
+ *
+ * Extends the Symfony All constraint to only validate after install.
+ */
+#[Constraint(
+  id: 'AfterInstall',
+  label: new TranslatableMarkup('After install composite constraint', [], ['context' => 'Validation']),
+  type: FALSE
+)]
+class AfterInstallConstraint extends Sequentially {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(mixed $constraints = NULL, ?array $groups = NULL, mixed $payload = NULL) {
+    $constraint_manager = \Drupal::service('validation.constraint');
+
+    // Instantiate constraint objects for array definitions.
+    $constraint_objects = [];
+    foreach ($constraints as $id => $options) {
+      if (!is_object($options)) {
+        $constraint_objects[] = $constraint_manager->create($id, $options);
+      }
+      else {
+        $constraint_objects[] = $options;
+      }
+    }
+    parent::__construct($constraint_objects, $groups, $payload);
+  }
+
+}