NULL Pointer Dereference in REST API properties_parse via Malformed user_properties

In nanomq versions 0.24.11 and earlier, a NULL pointer dereference in `properties_parse()` allows an authenticated attacker to crash the NanoMQ broker by sending a POST request to `/api/v4/mqtt/publish` with `user_properties` as a JSON array instead of a JSON object. The crash occurs because `strlen()` is called on a NULL `item->string` pointer when iterating over array elements. An authenticated attacker can exploit this to crash the NanoMQ broker process. This is patched in version 0.24.14.