Cellular: Fix compile warnings for CellularNetwork

Author: Mirela Chirica

features/cellular/framework/API/CellularNetwork.h

@@ -150,9 +150,9 @@ class CellularNetwork : public NetworkInterface {
             op_status = Unknown;
             op_rat = RAT_UNKNOWN;
             next = NULL;
-            op_long[0] = NULL;
-            op_short[0] = NULL;
-            op_num[0] = NULL;
+            op_long[0] = '\0';
+            op_short[0] = '\0';
+            op_num[0] = '\0';
         }
     };