Use nullptr for empty list of event handlers
Use nullptr for empty list of event handlers

Summary: This avoids copying a `shared_ptr` to an empty vector in cases when the client/server have uninstalled all handlers. Note that `ContextStack` already correctly handles the case when the `shared_ptr<vector<TProcessorEventHandler>>` member is `nullptr`, so no corresponding changes are needed there.

Reviewed By: vitaut

Differential Revision: D18645204

fbshipit-source-id: d855d68a6ba1a08dc4b503676d850195a32a3e5b

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