chore: prevent null in heritage extends expressions

[Emyrk]

This is a temporary workaround to generate valid typescript. This solution is not completely correct, and is subject to future change.

Related to #40

This solution treats a pointer the same as without a pointer. The outcome is therefore a subset of the valid types, so we can expand the output type later, and it should be backwards compatible.

type A struct {
	Foo string
}

type B struct {
	*A
	Bar int
	Qux bool
}

interface A {
    Foo: string;
}

// Ideally 'extends A' would also accept 'A' to be `{}`.
// I was really hoping we could do something like 'extends A | {}', but alas we cannot
interface B extends A {
    Bar: number;
    Qux: boolean;
}

[toqueteos]

Oh nice touch doing the loop. I didn't consider pointers could be multiple level