Handle nil arrays in autogenerated RPC wrappers

[fyfyrchik]

Is your feature request related to a problem? Please describe.

RPC binding generator doesn't expect nil when handling a slice

neo-go/pkg/smartcontract/rpcbinding/binding.go

Line 700 in 6d20772

arr, ok := item.Value().([]stackitem.Item)

But, compiler emits NULL by default for slice fields in struct

neo-go/pkg/compiler/codegen.go

Line 346 in 6d20772

case *types.Struct:

This is inconsistent and prevents me from using autogenerated named types.
The problem may also not be immediately obvious, as it is hard to guess when sth in contract is Null and when an empty array.

Describe the solution you'd like

Allow to have nils in struct slice fields parsed with FromStackItem().

Describe alternatives you've considered

1. Have a configuration option (nullable slice) for this.
2. Change compiler to respect the declared ABI (may be done separately).