Separate nullsafe modify assertion into its own function

The previous non-nullsafe call narrows the type of $d, which could
affect the nullsafe assertion on the next line.

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

Author: phpstan-bot

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

@@ -46,6 +46,9 @@ function (\DateTimeImmutable $dt, string $s): void {
 
 function (?\DateTimeImmutable $d): void {
 	assertType('DateTimeImmutable', $d->modify('+1 day'));
+};
+
+function (?\DateTimeImmutable $d): void {
 	assertType('DateTimeImmutable|null', $d?->modify('+1 day'));
 };