Incorrect inferred type with allowUndefinedOverrides: false

```ts
import { merge } from "ts-deepmerge";

const a: { o: string } = { o: "Hello" };

const b: { o?: string } = {};

const c = merge.withOptions({ allowUndefinedOverrides: false }, a, b);

// this is incorrect
//   c is of type { o: string | undefined }
// but should be
//   { o: string }
// since c.o can never be undefined
```
