Filters should validate for correct data using the field class' clean() function

[sjamaan]

When a uuid field is being filtered and the frontend passes in the empty string, this triggers a 500 internal server error on the backend.

Binder tries to prevent this with various regex and other kinds of checks, and we could of course add something like that specifically for uuids (and maybe we should? uuids aren't strings), but more generally I think it's a good idea to use the field class to clean the value, if possible.