fix: recreate compat field, of null pubkeys in multisig (cosmos#10515)

<!--
The default pull request template is for types feat, fix, or refactor.
For other templates, add one of the following parameters to the url:
- template=docs.md
- template=other.md
-->

## Description

Closes: cosmos#10042

Port cosmos#10275 to master. This is addressing a regression issue for the previously merged cosmos#10061. Changelog entry and tests included in the merged cosmos#10061.
---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow-up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

crypto/keys/multisig/amino.go

@@ -78,11 +78,28 @@ func (m *LegacyAminoPubKey) UnmarshalAminoJSON(tmPk tmMultisig) error {
 	// Instead of just doing `*m = *protoPk`, we prefer to modify in-place the
 	// existing Anys inside `m` (instead of allocating new Anys), as so not to
 	// break the `.compat` fields in the existing Anys.
-	m.PubKeys = make([]*types.Any, len(protoPk.PubKeys))
+	if m.PubKeys == nil {
+		m.PubKeys = make([]*types.Any, len(tmPk.PubKeys))
+	}
 	for i := range m.PubKeys {
-		m.PubKeys[i] = &types.Any{}
-		m.PubKeys[i].TypeUrl = protoPk.PubKeys[i].TypeUrl
-		m.PubKeys[i].Value = protoPk.PubKeys[i].Value
+		if m.PubKeys[i] == nil {
+			// create the compat jsonBz value
+			bz, err := AminoCdc.MarshalJSON(tmPk.PubKeys[i])
+			if err != nil {
+				return err
+			}
+
+			m.PubKeys[i] = protoPk.PubKeys[i]
+			// UnmarshalJSON():
+			// just sets the compat.jsonBz value.
+			// always succeeds: err == nil
+			if err := m.PubKeys[i].UnmarshalJSON(bz); err != nil {
+				return err
+			}
+		} else {
+			m.PubKeys[i].TypeUrl = protoPk.PubKeys[i].TypeUrl
+			m.PubKeys[i].Value = protoPk.PubKeys[i].Value
+		}
 	}
 	m.Threshold = protoPk.Threshold