Management of null values
- EditableGrid::getRowField respects null values
- getTypedValue respects null values
- NumberCellEditor.editorValue treats null as NaN (empty textfield)
- NumberCellRenderer.render treats null as NaN (nan symbol)
- SelectCellEditor.isValueSelected treats null and '' as equal since
  . if a PHP associative array is used for the column values, the key will never be null)
  . option.value can never be null neither
- CellRenderer._render respects null
- EnumCellRenderer.getLabel handles null labels as ''
- MultiselectCellRenderer/Editor: null treated as an empty list