fix(example): Check if readings are not null

Author: P-R-O-C-H-Y

libraries/Zigbee/examples/Zigbee_On_Off_Switch/Zigbee_On_Off_Switch.ino

@@ -143,8 +143,14 @@ void setup() {
       "IEEE Address: %02X:%02X:%02X:%02X:%02X:%02X:%02X:%02X\r\n", device->ieee_addr[7], device->ieee_addr[6], device->ieee_addr[5], device->ieee_addr[4],
       device->ieee_addr[3], device->ieee_addr[2], device->ieee_addr[1], device->ieee_addr[0]
     );
-    Serial.printf("Light manufacturer: %s\r\n", zbSwitch.readManufacturer(device->endpoint, device->short_addr, device->ieee_addr));
-    Serial.printf("Light model: %s\r\n", zbSwitch.readModel(device->endpoint, device->short_addr, device->ieee_addr));
+    char* manufacturer = zbSwitch.readManufacturer(device->endpoint, device->short_addr, device->ieee_addr);
+    char* model = zbSwitch.readModel(device->endpoint, device->short_addr, device->ieee_addr);
+    if(manufacturer != nullptr) {
+      Serial.printf("Light manufacturer: %s\r\n", zbSwitch.readManufacturer(device->endpoint, device->short_addr, device->ieee_addr));
+    }
+    if(model != nullptr) {
+      Serial.printf("Light model: %s\r\n", zbSwitch.readModel(device->endpoint, device->short_addr, device->ieee_addr));
+    }
   }
 
   Serial.println();