btif_hf: Fix NULL phone number

Bug: 231535626
Test: cert/run
QA testing before submit

Change-Id: I2be64c3db39e35ff4b31de4714634e77670338c3

system/btif/src/btif_hf.cc

@@ -1075,7 +1075,7 @@ bt_status_t HeadsetInterface::ClccResponse(
   if (index == 0) {
     ag_res.ok_flag = BTA_AG_OK_DONE;
   } else {
-    std::string cell_number(number);
+    std::string cell_number(number ? number : "");
     BTIF_TRACE_EVENT(
         "clcc_response: [%d] dir %d state %d mode %d number = %s type = %d",
         index, dir, state, mode, PRIVATE_CELL(cell_number), type);