You can’t. Struct are considered value types, and by definition can’t be null. The easiest way to make it nullable is to make it a reference type.
The answer you need to ask yourself is “Why is this a struct?” and unless you can think of a really solid reason, don’t, and make it a class. The argument about a struct being “faster”, is really overblown, as structs aren’t necessarily created on the stack (you shouldn’t rely on this), and speed “gained” varies on a case by case basis.
See the post by Eric Lippert on the class vs. struct debate and speed.