Display 'nothing' for the empty union type

[ahejlsberg]

With this PR we display the name 'nothing' for empty union types. Empty union types can occur when control flow type analysis concludes that there is no possible type for a variable that matches the preceding type guards in a particular code path. For example:

function foo(x: string | number) {
    if (typeof x === "string") {
        x;  // x is of type string here
    }
    else if (typeof x === "number") {
        x;  // x is of type number here
    }
    else {
        x;  // x is of type nothing here
    }
}

The 'nothing' type cannot be explicitly named, it has no members, and no other types are compatible with it.

[mhegazy]

👍