Add regression test for #11102

Closes phpstan/phpstan#11102

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-11102.php

@@ -0,0 +1,15 @@
+<?php declare(strict_types = 1);
+
+namespace Bug11102;
+
+use DateTime;
+use function PHPStan\Testing\assertType;
+
+/** @var array{'start': DateTime|null} $details */
+$details = ['start' => null];
+
+$startIsADateTime = $details['start'] instanceof DateTime;
+
+if ($startIsADateTime) {
+	assertType('DateTime', $details['start']);
+}