Execution result should be null

[zajko]

To make the json RPC more consistent all occurrences of skip_serializing_if for Option type were removed. As a general rule we return 'null' for None and the three exceptions from json RPC are inconsistencies. List of fields that were of type Option<...> and had skip_serializing_if:

• GetTransactionResult::execution_info
• GetDeployResult::execution_info
• Error::data (in json_rpc submodule)