Update MifareClassicValueBlock.ino

Updated code to account for serial behavior change in ATMEGA32U4

examples/MifareClassicValueBlock/MifareClassicValueBlock.ino

@@ -43,6 +43,7 @@ MFRC522::MIFARE_Key key;
  */
 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