String fakePayload1 = "12,34,567,89,1011,12,131415";
String fakePayload2 = "999";
String fakePayload3 = "";
String fakePayload4 = ",,,3";

void extractData(String & message) {
  String extraction;
  int startPos = 0;
  int length;

  Serial.print(F("\nAnalysing [")); Serial.print(message); Serial.println(F("]"));

  do {
    length = message.substring(startPos).indexOf(',');
    if (length >= 0) {
      extraction = message.substring(startPos, startPos + length);
      startPos = startPos + length + 1;
    } else {
      extraction = message.substring(startPos);
    }
    Serial.print(F("extracted : [")); Serial.print(extraction); Serial.println(F("]"));
  } while (length >= 0);
}

void setup() {
  Serial.begin(115200); Serial.println();
  extractData(fakePayload1);
  extractData(fakePayload2);
  extractData(fakePayload3);
  extractData(fakePayload4);
}

void loop() {}