empty method for javascript

(function( $ ){
  $.fn.blank = function() {
    return $(this).val() == ""
  };
})( jQuery );

Usage

if ($("#old_password").blank()) {
  console.log("old passworld is blank");
}

Better use

$("#old_password").is(":blank")

if you dont have :blank available to your version of jquery (I didn't have it in 1.6) add it like this:

$.extend($.expr[':'],{
    blank: function(element) {
        return $.trim($(element).val()) === '';
    }
});

Thanks to @ethangarofolo for reminding me to use threequals 
Thanks to @tehviking for suggestion of trim 
Thanks to @cowboyd and the link to http://james.padolsey.com/javascript/extending-jquerys-selector-capabilities/ to create the custom selector