Editor "list": emptyValue:null allow to keep current value in the cell

[KES777]

Is your feature request related to a problem? Please describe.*
In my table I do not have lookup values for some IDs. This happened because of inconsistent database: data was not migrated properly or so.

When I try to edit this cell and then close list I do not want to lose the current value.

Describe the solution you'd like
I would like if null will be interpreted as do nothing. (Or, to not break current behavior, undefined should be interpreted as do nothing).

Describe alternatives you've considered
emptyValue: function( cell ) { return cell.getValue() }