fix: resolve PHPStan static analysis warnings

app/Services/PlanService.php

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