By default, the filter values don’t include empty values, even if the selected column in the table does include empty values. Is there a setting I missed to be able to select (and filter) on empty values?
See https://docs.webix.com/samples/60_pro/01_datatable/03_filters/06_filter_multiselect.html for reference.
you can replace null or empty strings ("") with space (" "). then system will accept them as object and include in select list.
you can do this in scheme->$init or custom datatype.
Please check also this topic.
In short, you can
But the solution in the above topic is related to single-option filters (select, richselect). Multiselect has a different output format (values for comparison), so the compare method will look as https://webix.com/snippet/f712beb9