Fix Core::hasCallback to return true for State::Empty
Fix Core::hasCallback to return true for State::Empty

Summary:
This specifically fixes a TSAN detected race where SemiFuture destructor was checking Core state to see if deferred executor has to be detached, while that state could become Empty if the callback is being proxied and both threads code be racing while accessing the executor.
hasCallback() checks if the callback was ever set (not if the callback is stil alive) and it's true for State::Empty (similar to State::Done).

Differential Revision: D24271346

fbshipit-source-id: 5f52f20783d36745d1200497e0300bc086b65213

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