Fix first bellatrix block when using mev-boost

[terencechain]

Caught during e2e test with builder

When requesting a builder block while head is at Altair. The proposer would have packed a nil header block which causes panic during state root transition.

Returning nil is also dangerous, we should return error instead

This would have only affected the first proposed block using mev-boost on bellatrix, we are far passed that point
and luckily that relayer disabled itself until the first Bellatrix finalized checkpoint so we did not encounter this run time