## Description
Ref: https://github.com/cosmos/cosmos-sdk/issues/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](4a6a1e3cb8/x/auth/tx/builder.go (L208)) messages are coerced to an any type in [NewAnyWithValue](4a6a1e3cb8/codec/types/any.go (L61)). 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](88909d6f2b/math/dec.go (L801)) implementation.
This side effect *changes* the output of a JSON marshal before and after the call to Dec.Marshal, as shown below.
```go
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.
---
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