Add assertType for nullsafe modify call in date-format test

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

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/date-format.php

@@ -46,4 +46,5 @@ function (\DateTimeImmutable $dt, string $s): void {
 
 function (?\DateTimeImmutable $d): void {
 	assertType('DateTimeImmutable', $d->modify('+1 day'));
+	assertType('DateTimeImmutable|null', $d?->modify('+1 day'));
 };