单选按钮不能为空

这是我的单选按钮代码。我想添加条件，当一个单选按钮没有被用户选中，然后它会出来警告有数据空。完成后，它将计算sum。

<form action="" method="post">
Player 1:
male <input type="radio" name="gender[1]" value="1">
female <input type="radio" name="gender[1]" value="2">
<br>
Player 2:
male <input type="radio" name="gender[2]" value="1">
female <input type="radio" name="gender[2]" value="2">
<br>
Player 3:
male <input type="radio" name="gender[3]" value="1">
female <input type="radio" name="gender[3]" value="2"> 
<br>
<input type="submit" name="submit" value="submit">
</form>
<?php
if (isset($_POST['submit']))
{
    $sum=0;
    if (isset($_POST['gender']))
    {
        $gender=$_POST['gender'];
        foreach ($gender as $value)
            {
               $sum=$sum+$value;
            }
            echo $sum;
    }
    else
    {
        echo 'you did not choose any genders';
    }
}
?>