GattClient: Improve test of the absence of iterator.

Author: pan-

BLE_GattClient/main.cpp

@@ -259,7 +259,7 @@ class GattClientProcess : private mbed::NonCopyable<GattClientProcess> {
      */
     void process_next_characteristic(void)
     {
-        if (_it == 0) {
+        if (!_it) {
             _it = _characteristics;
         } else {
             _it = _it->next;