Summary: [Folly] Support various operations on empty `IPAddress` and `SocketAddress`, i.e. default-constructed instances, such as `operator==` and `hash`. V.s. throwing exceptions.

Reviewed By: simpkins

Differential Revision: D15750100

fbshipit-source-id: 89853635f6aab3c295f6fca4e66e6435b0839357