bug: forge config with default values does not dump proper optimizer / optimizer_runs

[grandizzy]

          Hi folks @yash-atreya @zerosnacks @grandizzy, it looks like the change of `optimizer` to an `Option<bool>` also changed the format of on-disk hardhat-style build-info json artifacts, which affects external tooling (e.g. https://github.com/crytic/crytic-compile/issues/581). Was that change intentional?

Originally posted by @elopez in #9673 (comment)

With default settings forge config --json doesn't show optimizer = false but

forge config --json | grep optimizer
  "optimizer": null,
  "optimizer_runs": null,
  "optimizer_details": null,

forge config doesn't include optimizer = false either.

If optimizer = true specified in foundry.toml then proper values are dumped

forge config --json | grep optimizer
  "optimizer": true,
  "optimizer_runs": 200,
  "optimizer_details": null,

forge config | grep optimizer
optimizer = true
optimizer_runs = 200