Return non-nil RPC results for 0 length slices.

If a JSON array result was successfully calculated, but the
slice/array is empty, the result must be marshaled as '[]' rather than
the JSON null value.  To do this in go, the RPC handlers should never
return nil slices for non-error returns, but return a non-nil slice
header with 0 length.

For example, an empty listtransactions result should be returned as
[]btcjson.ListTransactionsResult{}, rather than nil.