Avoid Uncaught TypeError

When JSON contains record with null or undefined element,
Sort by string will fail with `Uncaught TypeError: Cannot read property 'toLowerCase' of null `
And dynatable process will hang up.

This PR fix that error.

I know that we can use custom sort function. But it would be nice if dynatable handle unexpected input.
http://jsfiddle.net/georgeosddev/cR3p8/2/