Fix normalize parameters

Why is empty string not a valid value for parameter?
What if I really want to calculate string length on server? %)
```js
var method = new ApiMethod({
    name : 'getLength',
    params : { str : { required : true }},
    action : function(params) { return params.str.length; }
});
method.exec({ str : '' }); // missing str parameter =(
```