Update rfid_default_keys.ino

Updated code to account for serial behavior change in ATMEGA32U4

examples/rfid_default_keys/rfid_default_keys.ino

@@ -53,6 +53,7 @@ byte knownKeys[NR_KNOWN_KEYS][MFRC522::MF_KEY_SIZE] =  {
  */
 void setup() {
     Serial.begin(9600);         // Initialize serial communications with the PC
+    while (!Serial);            // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
     SPI.begin();                // Init SPI bus
     mfrc522.PCD_Init();         // Init MFRC522 card
     Serial.println("Try the most used default keys to print block 0 of a MIFARE PICC.");