Optional values can not be unset after setting

[aicam]

When an optional value is set in an Operator (like SeperateBy Column in histogram), if you select empty string, it gives an error: "undefined" is no longer a valid option. It should allow to remove the option since its optional. If you do not set it from the beginning, it works fine, but if you set it, you need to remove the whole operator first to be able to unset that variable.