stmgr: GetNtwkVersion: return genesis network version before the firs…

…t upgrade

chain/stmgr/forks.go

@@ -8,6 +8,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/filecoin-project/lotus/build"
+
 	"github.com/filecoin-project/specs-actors/v7/actors/migration/nv15"
 
 	"github.com/ipfs/go-cid"
@@ -161,7 +163,8 @@ func (us UpgradeSchedule) GetNtwkVersion(e abi.ChainEpoch) (network.Version, err
 			return u.Network, nil
 		}
 	}
-	return network.Version0, xerrors.Errorf("Epoch %d has no defined network version", e)
+
+	return build.GenesisNetworkVersion, nil
 }
 
 func (sm *StateManager) HandleStateForks(ctx context.Context, root cid.Cid, height abi.ChainEpoch, cb ExecMonitor, ts *types.TipSet) (cid.Cid, error) {