To my understanding, multiple select does not allow select none, it has automatically select the first one unless you assign some other value in its 'Value' properties.
As a workaround, you can assign an empty item in the first selection and the listbox will automatically select the first one unless the user select some other item.
By doing that, the default selection, which is the empty one, can simulate select none in your case.
figure;uicontrol('Style','listbox',...
'String',{'','rr','tt','gg'},... % Assign an empty one as the first selection
'position',[10,20,100,200],...
'min',0,...
'max',1)
