VersionedBeaconBlock IsEmpty() missing blocks

[olegshmuelov]

IsEmpty() not working properly for both VersionedBeaconBlock/VersionedBlindedBeaconBlock

• Deneb blocks is not being checked for nil.
  

  go-eth2-client/spec/versionedbeaconblock.go

  Line 38 in 97e8ba2

  return v.Phase0 == nil && v.Altair == nil && v.Bellatrix == nil && v.Capella == nil

• Capella&Deneb blocks are not being checked for nil.
  

  go-eth2-client/api/versionedblindedbeaconblock.go

  Line 37 in 97e8ba2

  return v.Bellatrix == nil