fix(math): Dec marshal shouldn't have side effects

[kocubinski]

Description

Ref: #15170
Included in: #15515

Required to resolve a discrepancy between the sign mode being implemented in #15170 and the existing legacy one.

When calling tx builder SetMsgs messages are coerced to an any type in NewAnyWithValue. This marshals a message as proto bytes. If a message has cosmos.Dec typed fields, and they are set to nil, the will be set to 0 during seriazliation in the Dec.Marshal implementation.

This side effect changes the output of a JSON marshal before and after the call to Dec.Marshal, as shown below.

rates := &stakingtypes.CommissionRates{}
MarshalJson(rates)
// {"rate":"0","max_rate":"0","max_change_rate":"0"}

MarshalProto(rates)

MarshalJson(rates)
//{"rate":"0.000000000000000000","max_rate":"0.000000000000000000","max_change_rate":"0.000000000000000000"}

An integration is test is also committed asserting the (now fixed) behavior above. Its asserts should be changed once a release of math is made and /tests/go.mod is updated.

---

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...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• 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 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)

[julienrbrt]

lgtm!

[julienrbrt]

This should be under unreleased.

[JeancarloBarrios]

Lgtm