fix: prevent nil DecCoin creation when converting Coins to DecCoins (#12903)

## Description

Closes: #12902 



---

### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [ ] 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
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/main/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

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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)

CHANGELOG.md

@@ -117,6 +117,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (x/auth/tx) [#12474](https://github.com/cosmos/cosmos-sdk/pull/12474) Remove condition in GetTxsEvent that disallowed multiple equal signs, which would break event queries with base64 strings (i.e. query by signature).
 * (store/rootmulti) [#12487](https://github.com/cosmos/cosmos-sdk/pull/12487) Fix non-deterministic map iteration.
 * (x/group) [#12888](https://github.com/cosmos/cosmos-sdk/pull/12888) Fix event propagation to the current context of `x/group` message execution `[]sdk.Result`.
+* (sdk/dec_coins) [#12903](https://github.com/cosmos/cosmos-sdk/pull/12903) Fix nil `DecCoin` creation when converting `Coins` to `DecCoins`
 * (x/upgrade) [#12906](https://github.com/cosmos/cosmos-sdk/pull/12906) Fix upgrade failure by moving downgrade verification logic after store migration.
 
 ### Deprecated

types/dec_coin.go

@@ -182,10 +182,14 @@ func sanitizeDecCoins(decCoins []DecCoin) DecCoins {
 // NewDecCoinsFromCoins constructs a new coin set with decimal values
 // from regular Coins.
 func NewDecCoinsFromCoins(coins ...Coin) DecCoins {
-	decCoins := make(DecCoins, len(coins))
+	if len(coins) == 0 {
+		return DecCoins{}
+	}
+
+	decCoins := make([]DecCoin, 0, len(coins))
 	newCoins := NewCoins(coins...)
-	for i, coin := range newCoins {
-		decCoins[i] = NewDecCoinFromCoin(coin)
+	for _, coin := range newCoins {
+		decCoins = append(decCoins, NewDecCoinFromCoin(coin))
 	}
 
 	return decCoins

types/dec_coin_test.go

@@ -569,6 +569,29 @@ func (s *decCoinTestSuite) TestNewDecCoinsWithIsValid() {
 	}
 }
 
+func (s *decCoinTestSuite) TestNewDecCoinsWithZeroCoins() {
+	zeroCoins := append(sdk.NewCoins(sdk.NewCoin("mytoken", sdk.NewInt(0))), sdk.Coin{Denom: "wbtc", Amount: sdk.NewInt(10)})
+
+	tests := []struct {
+		coins        sdk.Coins
+		expectLength int
+	}{
+		{
+			sdk.NewCoins(sdk.NewCoin("mytoken", sdk.NewInt(10)), sdk.NewCoin("wbtc", sdk.NewInt(10))),
+			2,
+		},
+		{
+			zeroCoins,
+			1,
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		s.Require().Equal(sdk.NewDecCoinsFromCoins(tc.coins...).Len(), tc.expectLength)
+	}
+}
+
 func (s *decCoinTestSuite) TestDecCoins_AddDecCoinWithIsValid() {
 	lengthTestDecCoins := sdk.NewDecCoins().Add(sdk.NewDecCoin("mytoken", sdk.NewInt(10))).Add(sdk.DecCoin{Denom: "BTC", Amount: math.LegacyNewDec(10)})
 	s.Require().Equal(2, len(lengthTestDecCoins), "should be 2")