fix: Make MulDecTruncate() return nil DecCoins when multiplier is 0 (#10687)

## Description
The `DecCoins.MulDecTruncate` function is supposed to panic instead of returning `nil` when multiplier `d` is zero. This fixes that behavior and updates the related test `decCoinTestSuite.TestDecCoins_MulDecTruncate` to expect a panic when multiplier `d` is zero.

Closes: #9790 

---

### Author Checklist

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- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
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- [x] 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)
- [x] 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

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- [ ] 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
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- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -189,6 +189,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * [\#10593](https://github.com/cosmos/cosmos-sdk/pull/10593) Update swagger-ui to v4.1.0 to fix xss vulnerability.
 * [\#10674](https://github.com/cosmos/cosmos-sdk/pull/10674) Fix issue with `Error.Wrap` and `Error.Wrapf` usage with `errors.Is`.
 * [\#10897](https://github.com/cosmos/cosmos-sdk/pull/10897) Fix: set a non-zero value on gas overflow.
+* [#9790](https://github.com/cosmos/cosmos-sdk/pull/10687) Fix behavior of `DecCoins.MulDecTruncate`.
 
 ### State Machine Breaking
 

types/dec_coin.go

@@ -373,12 +373,15 @@ func (coins DecCoins) MulDec(d Dec) DecCoins {
 }
 
 // MulDecTruncate multiplies all the decimal coins by a decimal, truncating. It
-// panics if d is zero.
+// returns nil DecCoins if d is zero.
 //
 // CONTRACT: No zero coins will be returned.
 func (coins DecCoins) MulDecTruncate(d Dec) DecCoins {
-	var res DecCoins
+	if d.IsZero() {
+		return DecCoins{}
+	}
 
+	var res DecCoins
 	for _, coin := range coins {
 		product := DecCoin{
 			Denom:  coin.Denom,

types/dec_coin_test.go

@@ -880,12 +880,10 @@ func (s *decCoinTestSuite) TestDecCoins_MulDecTruncate() {
 	}{
 		{"No Coins", sdk.DecCoins{}, sdk.NewDec(1), sdk.DecCoins(nil), false},
 
-		// TODO - Fix test - Function comment documentation for MulDecTruncate says if multiplier d is zero, it should panic.
-		// However, that is not the observed behaviour. Currently nil is returned.
 		{"Multiple coins - zero multiplier", sdk.DecCoins{
 			sdk.DecCoin{testDenom1, sdk.NewDecWithPrec(10, 3)},
 			sdk.DecCoin{testDenom1, sdk.NewDecWithPrec(30, 2)},
-		}, sdk.NewDec(0), sdk.DecCoins(nil), false},
+		}, sdk.NewDec(0), sdk.DecCoins{}, false},
 
 		{"Multiple coins - positive multiplier", sdk.DecCoins{
 			sdk.DecCoin{testDenom1, sdk.NewDecWithPrec(15, 1)},