if statement, is it necessary for this particular code or is it redundant?

Could someone please tell me what is the function of if (month == null) { return monthNumber; } in the code below. I saw this code at Java tutorial Oracle. I have deleted the if statement above and the code below works just fine. Is the if statement unnecessary or it is included for some reason unknown to me.

public class StringSwitchDemo {

public static int getMonthNumber(String month) {

    int monthNumber = 0;

    if (month == null) {
        return monthNumber;
    }


    switch (month.toLowerCase()) {
        case "january":
            monthNumber = 1;
            break;
        case "february":
            monthNumber = 2;
            break;
        case "march":
            monthNumber = 3;
            break;
        case "april":
            monthNumber = 4;
            break;
        case "may":
            monthNumber = 5;
            break;
        case "june":
            monthNumber = 6;
            break;
        case "july":
            monthNumber = 7;
            break;
        case "august":
            monthNumber = 8;
            break;
        case "september":
            monthNumber = 9;
            break;
        case "october":
            monthNumber = 10;
            break;
        case "november":
            monthNumber = 11;
            break;
        case "december":
            monthNumber = 12;
            break;
        default: 
            monthNumber = 0;
            break;
    }

    return monthNumber;
}

public static void main(String[] args) {

    String month = "";

    int returnedMonthNumber =
        StringSwitchDemo.getMonthNumber(month);

    if (returnedMonthNumber == 0) {
        System.out.println("Invalid month");
    } else {
        System.out.println(returnedMonthNumber);
    }
}
}