Have ticker cipher creation take in an optional std::string for psk context
Have ticker cipher creation take in an optional std::string for psk context

Summary: D8786637 changed behavior for proxygen such that it would previously pass in an empty string for psk context where as after it would pass in nothing. We need to handle both cases (no psk context and intentional empty string) so we'll pass in the string as an optional instead

Reviewed By: knekritz

Differential Revision: D9724852

fbshipit-source-id: 810a7cc6ca7488c8a8f1a1c7db1111c5ab482c00

facebook/wangle,facebook/wangle,facebook/wangle