An NSNumber object containing a Boolean value that determines if an additional item which represents nil is inserted into a matrix or pop-up menu before the items in the content array.
Available in Mac OS X v10.4 and later.
Declaration Syntax Version Information- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)