fix(API): 입고처리 receiving_location 필드 null 처리

- nullable 필드 접근 시 undefined array key 에러 수정
- null 병합 연산자 적용

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Services/ReceivingService.php

@@ -248,7 +248,7 @@ public function process(int $id, array $data): Receiving
             $receiving->receiving_date = $data['receiving_date'] ?? now()->toDateString();
             $receiving->lot_no = $lotNo;
             $receiving->supplier_lot = $data['supplier_lot'] ?? null;
-            $receiving->receiving_location = $data['receiving_location'];
+            $receiving->receiving_location = $data['receiving_location'] ?? null;
             $receiving->receiving_manager = $data['receiving_manager'] ?? null;
             $receiving->status = 'completed';
             $receiving->remark = $data['remark'] ?? $receiving->remark;