DataTable isSelected check performance improvement

[Ketec]

[ x] bug report => Search github for a similar issue or PR before submitting
[ ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Current behavior

    isSelected(rowData) {
        if(rowData && this.selection) {
            if(this.dataKey) {

if this.selection is [], it does not return false. Empty array is still true. It may need a extra && this.selection.length > 0.
Expected behavior
It should properly return false if selection is empty. This may also affect performance because angular change detection calls this method for every row on every mouse click or even mouse move and it constantly triggers all the logic inside (a grid with a thousand rows could easily trigger it tens of thousands of times every second).

[cagataycivici]

This broke the table so reverted.