fix(bootstrap): deserialize null as `f64::NAN`

When doing optimized build through opt-dist,
I've often run into errors like `invalid type: null, expected f64`.
This is likely because some f64 fields might actually bet set null.
Unfortunately, serde_json doesn't handle null <-> NaN well.

This commit addresses it by having a custom deserialize method, so
null is always be deserialized to `f64:NAN`.

See:

* https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/opt-dist.3A.20.60invalid.20type.3A.20null.2C.20expect.20f64.60.20failure
* https://github.com/serde-rs/json/issues/202