Decoding of optional null values should not return null

[aaroncox]

An action (like eosio.msig::approve) containing an optional field (like proposal_hash) when decoded will return a null value as the value. This field should be omitted entirely.

Example action data for eosio.msig::approve

{
    proposer: 'foo',
    proposal_name: 'bar',
    level: {
        actor: 'baz',
        permission: 'active',
    },
}

Encoding this action will result in a proper encoded hex value - but when decoding the action again, it will result in a different action structure:

{
    proposer: 'foo',
    proposal_name: 'bar',
    level: {
        actor: 'baz',
        permission: 'active',
    },
    proposal_hash: null
}

This causes problems with the wharfkit/wallet-plugin-cleos representation of the transaction, since when you submit the above action as a cleos command, cleos will reject it with null being an invalid value.

The serializer in this instance, when decoding an action, will need to omit proposal_hash from the JSON-like representation of the action, instead of including it and setting it to a null value.

At this point I am unsure if omitting the field during decoding will break anything else, so some research is going to be required to determine the best course of action here.