Updated line 26 for null array, invalid arg errors

Need to check if ```'$args['testimonial_category']``` isset, otherwise with PHP debugging on you'll get the errors below:

```Warning: array_keys() expects parameter 1 to be array, null given in .../wp-content/plugins/clean-testimonials/class.testimonials-widget.php on line 26 
Warning: implode(): Invalid arguments passed in .../wp-content/plugins/clean-testimonials/class.testimonials-widget.php on line 26```