radiobuttons

Radio buttons - null option

Hi,

I am wondering if it is (already?) acceptable to omit the explicit
null option in a radio button control?
I have used radiobuttons for a non-required control. To apply the
null option the user would have to 'unselect' the one selected
radio button (this to save place by not adding a fourth radio button
or to not have to use a drop down control).
I suppose it is kind of an unusual behavior but it seems to me that
is is learnable, and I have seen it in product design. Has this kind
of behavior been introduced in forms before?

Thanks for your opinions,
Emely