Fix bug: Programmatic filter() was ignored for empty filter expression

* Execute filter() also when the provided String-based filter is empty
* Make getFilterPredicate() private: Subclasses should override
  filter() and getFilterExpression() instead
* Note: Keeping explicit filterValue argument of filter()
   - It is initialized using getFilterExpression() in
     getFilterPredicate())
   - This allows to change the filter expression when filter() invokes
     itself recursively (e.g., when filtering trees)

Issue-Ref: 4071
Issue-Url: https://af3-developer.fortiss.org/issues/4071



Signed-off-by: Simon Barner <barner@fortiss.org>