ContextStack with empty handlers speedup
ContextStack with empty handlers speedup

Summary:
If we see that the (shard_ptr<>&) handlers param has no entries, we should
treat the param as if it were nullptr, so that we don't have to atomic
inc/dec the shared_ptr reference.

(ContextStack handles handlers_ == nullptr just fine).

Reviewed By: jmswen

Differential Revision: D15403778

fbshipit-source-id: 3df3ff3b8b71ada311116f8d857957f71098a64b

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