Handling of not required inputs

[dietzc]

Two problems:

• If an input of a Plugin is required=false the default value should be set in the dialog component. if the default value is null, the user should be able to activate/deactivate this dialog component, in the node dialog, as we want to use the default values which are then determined by the Plugin somewhere in the code.
• If the required=false input of the Plugin is a column-selection, the <None> option should be selectable.