Adjust the epoch0 EBB construction to follow the consensus era

[dcoutts]

When we start a chain from a genesis in the OBFT era, rather than the Original era, the epoch zero EBB must still exist, even though all other epoch EBBs do not exist.

That EBB ought to have an empty slot leader schedule, otherwise we cannot have new non-legcay nodes create the same EBB, since new nodes do not implement the logic for Original era leader schedules. This does not affects any existing chain, since existing chains all start in the Original era and go through a hard fork to enter the OBFT era. This change only affects new test/dev chains that start in the OBFT era. This is needed to test mixed clusters of old/new nodes in a convenient way, that all start from genesis in the OBFT era.

[dcoutts]

bors r+

[iohk-bors[bot]]

Build succeeded

• ci/hydra:Cardano:cardano-sl:required
• buildkite/cardano-sl