Skip validations when required set to false and value is null or undefined

[asheriff]

When required is set to false the other validators on the attrubute are only run if the new attribute value is not null or undefined.

var Foo = Backbone.Model.extend({
  validate: {
    age: {
      type: 'number'
    }
  }
});

var foo = new Foo();
foo.set({blah: 'xyzzy'}); // => false
foo.isValid(); // => false


var Bar = Backbone.Model.extend({
  validate: {
    age: {
      type: 'number',
      required: false
    }
  }
});

var bar = new Bar();
bar.set({blah: 'xyzzy'}); // => bar
bar.isValid(); // => true
bar.set({age: 'old'}); // => false
bar.set({age: 1});
bar.isValid(); // => true