regex filter show only empty string

Question

I have made a java replica of excel's filter for jtables. It works fine, however, I can't get empty string filter to work with regexfilter, since adding "" to regexFilter would lead to show all results (also non-empty rows)

Empty cells are inserted in the table model as "" or " " via Vector.

Here's the current regex filter. Object table is the table inside the pop-up filter.

    for(int i =0;i< table.getModel().getRowCount();i++) {           
        String val = String.valueOf(table.getModel().getValueAt(i, 1));
        
        if(table.getModel().getValueAt(i, 0).equals(true) && (!val.equals("")|| !val.isBlank() || !val.isEmpty() || !val.equals(" "))) {
          
            keywordCheckBox.add(table.getModel().getValueAt(i, 1));
            filters.add(RowFilter.regexFilter((String)table.getModel().getValueAt(i, 1), column));
        }
        
        
        else if(table.getModel().getValueAt(i, 0).equals(true) && (val.equals("")|| val.isBlank() || val.isEmpty() || val.equals(" "))) {
           
            keywordCheckBox.add(table.getModel().getValueAt(i, 1));
            filters.add(RowFilter.regexFilter("/^$/", column));
        }

What's the regular expression that has to be added to regexfilter to show "" values and " " only?