unnecessary `Empty array` type in `wallet_getPermissions`'s schema

[huynhducduy]

Reference: https://github.com/starkware-libs/starknet-specs/blob/master/wallet-api/wallet_rpc.json#L48-L90
starknet-io/types-js#36

I think wallet_getPermissions method doesn't need the Empty array value at all, since we already have PERMISSION, and by default empty PERMISSION array will mean locked or not approved.

If we want to have explicit values for locked or not approved, we should use some other type, like false or null..., to avoid confusion.