Spread operator should declare members as undefined for null or undefined objects   

**TypeScript Version:**  2.1.4 & 2.2.0-dev.20161220 

**Code**
```ts
function test(t?: { a: number } | null): void {
    let b = { ...t };  // b: { a: number }
    let c: number = b.a;  // no error
}
```

**Expected behavior:**
`b.a` to be `number | undefined`, as if `t` is `null` or `undefined`, `b` is an empty object.
**Actual behavior:**
`b.a`is `number`.