Revert "fix: resolve PHPStan static analysis warnings"

This reverts commit 2d3e4b4a95.

app/Services/PlanService.php

@@ -74,9 +74,10 @@ class PlanService
 
         // 转换周期格式为新版格式
         $periodKey = self::getPeriodKey($period);
-        $price = $this->plan->prices[$periodKey] ?? null;
 
-        if ($price === null) {
+
+        // 检查价格时使用新版格式
+        if (!isset($this->plan->prices[$periodKey]) || $this->plan->prices[$periodKey] === NULL) {
             throw new ApiException(__('This payment period cannot be purchased, please choose another period'));
         }