One really quick way of doing this is defaulting to a choice in the UI. This makes sure that the value is non-empty because you can't "uncheck" a radio button.
<input type="radio" name="gender" id="optionsRadios1" value="female" checked> Female
Edit:
Write your own submit function, and bind it to your submit button's onclick()
function submit() {
if (!$("#optionsRadios1").prop("checked") && !$("#optionsRadios2").prop("checked")) {
alert("please select a gender");
}
else {
//Whatever you do to submit
}
}