Modify encoding to not output "i":null in JSON

[jwiegley]

PR checklist:

• Test coverage for the proposed changes
• PR description contains example output from repl interaction or a snippet from unit test output
• Documentation has been updated if new natives or FV properties have been added. To generate new documentation, issue cabal run tests. If they pass locally, docs are generated.
• Any changes that could be relevant to users have been recorded in the changelog
• In case of changes to the Pact trace output (pact -t), make sure pact-lsp is in sync.

Additionally, please justify why you should or should not do the following:

• Confirm replay/back compat
• Benchmark regressions
• (For Kadena engineers) Run integration-tests against a Chainweb built with this version of Pact

[jwiegley]

This change currently breaks 3 golden tests that depend on a particular hash of the txlogs. I just need to verify that the test simply needs to be updated.

[rsoeldner]

@jwiegley Where is this encoding being used, does it have to be forked?

[jwiegley]

@rsoeldner Yes, it will need a fork. It is used anywhere that Pact is serialized, such as modules in the db.