Add NullCoalesceRule test for phpstan/phpstan#13623

The original bug report was about a false positive "Offset ... on left side
of ??= always exists and is not nullable." error, so this adds a
non-regression test in NullCoalesceRuleTest.

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

Author: phpstan-bot

tests/PHPStan/Rules/Variables/NullCoalesceRuleTest.php

@@ -377,4 +377,9 @@ public function testBug13921(): void
 		]);
 	}
 
+	public function testBug13623(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13623.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/bug-13623.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug13623Rule;
+
+function (array $results): void {
+	$customers = [];
+
+	foreach ($results as $row) {
+		$customers[$row['customer_id']] ??= [];
+		$customers[$row['customer_id']]['orders'] ??= [];
+		$customers[$row['customer_id']]['orders'][$row['order_id']] ??= [];
+
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['balance_forward'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['new_invoice'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['payments'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['balance'] ??= $row['order_total'];
+	}
+};