Make FunctionRef support bool operator

Make FunctionRef support bool operator

Summary:
To support following code:
```
void foo(folly::FunctionRef<void(void)> callback = {}) {
  if (callback) {
    callback();
  }
}
```

Reviewed By: yfeldblum

Differential Revision: D4372296

fbshipit-source-id: 7d21e6a44b6f6b046b424f0139465511dbae7b8b