Update troubleshooting.rst

We have admin class with around 100 filters in it (yes, I know, it may sounds crazy, but filters are dynamically build and its very user-friendly. Also client wanted it :) ).
But when filter form is submitted - very long url is generated. I have created fast workaround for this issue.
Empty select/inputs just removed before form submit. 
It works with persisted filters as well (because non empty input/selects are marked).

@rande this code may be added for default template - it seems to be safe to use by default.