Skip serializing blob_schedule before Fulu

[michaelsproul]

Issue Addressed

Alternative to:

• fix: allow BLOB_SCHEDULE: [] field #7758

Proposed Changes

Serve the blob_schedule field on /eth/v1/config/spec only when Fulu is enabled. If the blob schedule is empty, we will still serve it as [], so long as Fulu is enabled.

Additional Info

This touches the same code as:

• Serve ConfigAndPreset for latest_stable fork #7777

But this PR is higher prio for Fusaka testing.

[mergify]

Some required checks have failed. Could you please take a look @michaelsproul? 🙏

[pawanjay176]

LGTM, fixed the test and merged unstable

[pawanjay176]

Actually this is quite annoying to deal with in the tests because a roundtrip ser/de isn't consistent. i.e. this test would fail

#[test]
    fn chain_spec_roundtrip() {
        let yamlconfig =
            ConfigAndPreset::from_chain_spec::<MainnetEthSpec>(&ChainSpec::mainnet(), None);

        let serialized = serde_yaml::to_string(&yamlconfig).unwrap();
        let deserialized: ConfigAndPreset = serde_yaml::from_str(&serialized).unwrap();
        assert_eq!(yamlconfig.config().blob_schedule, deserialized.config().blob_schedule);
    }

Not sure of a clean way to handle this other than setting fulu_fork_epoch all places where we do this kind of roundtrip.