org-export: Nil value from a filter means filter will be skipped

org-export: Nil value from a filter means filter will be skipped

* contrib/lisp/org-export.el (org-export-filter-apply-functions): Nil
  value from a filter means filter will be skipped. To ignore the
  current element or object, the filter has to return the empty string
  instead.

This change is done to ease filter writing. When writing
a backend-specific filter (and I guess most are), there's no more need
for the somewhat contrived:

  (if (not (eq backend 'e-latex)) data
    ... filter's job...)

A more straightforward:

  (when (eq backend 'e-latex)
    ... filter's job...)

is now enough.

On the other hand, it is not possible anymore to specify 'ignore as
a filter to ignore every element or object of a given type. To
achieve that goal, one can now write, for example:

  (add-to-list 'org-export-filter-example-block-functions
                 (lambda (value backend info)
                   (when (eq backend 'e-html) "")))

It will ignore every example block in the `e-html' export back-end.