Fix Herald to accept new JSON encoding of sparse arrays

Summary: JX.JSON was recently changed to use JSON.stringify (the native
implementation) if it is available. The native implementation has a behavioral
difference from the Javelin implementation, in that it does not compact sparse
arrays. Ignore nulls resulting from removals when processing the encoded action
and condition lists.
Test Plan: Removed conditions from Herald rules.
Reviewed By: aran
Reviewers: aran, cpojer, jungejason
CC: aran, epriestley
Differential Revision: 606