T.nullish

An absence primitive: a value that is the wrapped inner, null, or undefined. An ordinary schema, usable anywhere.

T.nullish(T.string); // string | null | undefined, uniform 1/3 each
T.nullish(inner).weighted({ null: 1, undefined: 1, value: 2 });

T.nullish is a dedicated three-way roll for the same reason T.optional is: T.nullable(T.undefinable(inner)) would give 50/25/25, not a uniform split.

For only null vs a value, use T.nullable. For only undefined vs a value, use T.undefinable.