jQuery.fn.offset: add a verification of arguments 'options'

Hi, I recently was learning jquery source code, I think this place needs a little modification.
```javascript
jQuery.fn.extend({
	offset: function( options ) {
		if ( arguments.length ) {
			return options === undefined ?
				this :
				this.each(function( i ) {
					jQuery.offset.setOffset( this, options, i );
				});
		}
...
```
If the 'options' is a empty object, I think 'offset' should return 'this'.
```javascript
jQuery.fn.extend({
	offset: function( options ) {
		if ( arguments.length ) {
			return options === undefined || jQuery.isEmptyObject( options ) ?
				this :
				this.each(function( i ) {
					jQuery.offset.setOffset( this, options, i );
				});
		}
...
```
As above, I added a method “ jQuery.isEmptyObject ” to verify the value of "options".