Use this constant as a dummy contract storage item prefix for those
situations when Billet is used as an MPT search wrapper. Otherwise it's
confusing to see `0` passed as a storage prefix to the billet
constructor and then panic inside the Billet's code if this prefix is
`0`.
Ref. https://github.com/nspcc-dev/neo-go/pull/2201.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>