allow construction of empty ProguardMap
allow construction of empty ProguardMap

Summary: I think there's no reason why this should not be default constructible. It makes sense for all of its field to be empty.

Reviewed By: ssj933

Differential Revision: D19838453

fbshipit-source-id: dc763f07c1d2317056c6b277457bc8fd1d624cb8

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