Optional fields deserialization in MsgEditSideChainValidator

[MarcinKustraCopper]

Summary of Bug

Recently I've been trying to integrate MsgEditSideChainValidator via Javascript SDK and Amino codec and I found interesting case with optional fields. E.g when I want to update Side Chain Validator:

type MsgEditSideChainValidator struct {
	Description   Description    `json:"description"`
	ValidatorAddr sdk.ValAddress `json:"address"`

	// We pass a reference to the new commission rate as it's not mandatory to
	// update. If not updated, the deserialized rate will be zero with no way to
	// distinguish if an update was intended.
	//
	// REF: #2373
	CommissionRate *sdk.Dec `json:"commission_rate"`

	SideChainId string `json:"side_chain_id"`
	// for SideFeeAddr, we do not update the values if they are not provided.
	SideFeeAddr []byte `json:"side_fee_addr"`

	SideConsAddr []byte `json:"side_cons_addr,omitempty"`
}

with only e.g. updated website, sending such amino-encoded payload:

{"type":"cosmos-sdk/MsgEditSideChainValidatorWithVoteAddr","value":{"address":"bva1...","description":{"details":"[do-not-modify]","identity":"[do-not-modify]","moniker":"[do-not-modify]","website":"foo.bar"},"side_chain_id":"chainId"}}

In this payload fields that I do not want to update were filled with nulls, yet they were skipped in JSON repr. Such payload also is used for signing purposes.
However, decoding this message on bnc-cosmos-sdk side and getting sign bytes yields completely different shape:

{"type":"cosmos-sdk/MsgEditSideChainValidatorWithVoteAddr","value":{"address":"bva1...","commission_rate":null,"description":{"details":"foo.bar","identity":"[do-not-modify]","moniker":"[do-not-modify]","website":"[do-not-modify]"},"side_chain_id":"chainId","side_fee_addr":null,"side_vote_addr":null}}

yielding in completely different sign data bytes.

Steps to Reproduce

As above.

Expected behaviour

Unified behaviour between js and cosmos SDKs.
I think that Golang SDK should drop nullable fields, since it is more flexible to add new fields without breaking compatibility with previous JS SDKs.
I submitted PR as part of this issue.

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