HtmlTable method: empty fails on empty table

Calling myTable.empty(); on an already empty HtmlTable you get this error

```Uncaught TypeError: Cannot call method 'isDisplayed' of undefined```

tracked it to the selectRange function for loop

```
for (var i = startRow; i <= endRow; i++){
			if (this.options.selectHiddenRows || rows[i].isDisplayed()) this[method](rows[i], true);
		}
```

This commit addresses the issue...


All the best
Daniel