bugfix: remove empty array elements to avoid wrong links on removed filters

when you have 3 or more filters and would like to remove one links are generated like:

http://yourdomain.com/category/filter1-filter2-/
http://yourdomain.com/category/filter1--filter3/

because there are still empty array elements left.
This fixes the bug and removes the empty elements from the array so that the links show up like:

http://yourdomain.com/category/filter1-filter2/
http://yourdomain.com/category/filter1-filter3/