fix(pos-app): handle empty response body in transactions proxy

The upstream API can return an empty body on error responses, causing
JSON.parse to fail. Read as text first and only parse if non-empty.

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

Author: ignaciosantise

dapps/pos-app/api/transactions.ts

@@ -61,7 +61,8 @@ export default async function handler(req: VercelRequest, res: VercelResponse) {
       headers: getApiHeaders(credentials.apiKey, credentials.merchantId),
     });
 
-    const data = await response.json();
+    const text = await response.text();
+    const data = text ? JSON.parse(text) : {};
 
     if (!response.ok) {
       return res.status(response.status).json(data);