jQuery add blank option to top of list and make selected to existing dropdown

So I have a dropdown list

<select id="theSelectId">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

This is what I would like

<select id="theSelectId">
  <option value="" selected="selected"></option>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

Trying to add a blank option before and set it to this, want to enforce the user to select a value from the original list but would like it to be blank when they see the option they have to choose.

Trying this but not working

// Add blank option
var blankOption = {optVal : ''};
$.each(blankOption, function(optVal, text) {
   $('<option></option>').val(optVal).html(text).preprendTo('#theSelectId');
});

and I have tried this but is clears out the other values

$('#theSelectId option').prependTo('<option value=""></option>');