fix: null guard for tx fee amounts (cosmos#10327)

## Description

It is possible to submit a TX with a fees object containing a Coin with a nil amount. This results in a rather cryptic redacted panic response when the basic validation checks fee Coins for negative amounts.

This PR adds an additional check for nil to provide a friendlier error message.

---

### 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 (note: No issue exists)
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing) (note: First PR against the SDK so please comment with what needs to be done)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [x] 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

@@ -112,6 +112,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Improvements
 
+* [\#10327](https://github.com/cosmos/cosmos-sdk/pull/10327) Add null guard for possible nil `Amount` in tx fee `Coins`
 * [\#9780](https://github.com/cosmos/cosmos-sdk/pull/9780) Remove gogoproto `moretags` YAML annotations and add `sigs.k8s.io/yaml` for YAML marshalling.
 * (x/bank) [\#10134](https://github.com/cosmos/cosmos-sdk/pull/10134) Add `HasDenomMetadata` function to bank `Keeper` to check if a client coin denom metadata exists in state.
 * (store) [\#10026](https://github.com/cosmos/cosmos-sdk/pull/10026) Improve CacheKVStore datastructures / algorithms, to no longer take O(N^2) time when interleaving iterators and insertions.

types/coin.go

@@ -144,6 +144,11 @@ func (coin Coin) IsNegative() bool {
 	return coin.Amount.Sign() == -1
 }
 
+// IsNil returns true if the coin amount is nil and false otherwise.
+func (coin Coin) IsNil() bool {
+	return coin.Amount.i == nil
+}
+
 //-----------------------------------------------------------------------------
 // Coins
 
@@ -590,6 +595,19 @@ func (coins Coins) IsAnyNegative() bool {
 	return false
 }
 
+// IsAnyNil returns true if there is at least one coin whose amount
+// is nil; returns false otherwise. It returns false if the coin set
+// is empty too.
+func (coins Coins) IsAnyNil() bool {
+	for _, coin := range coins {
+		if coin.IsNil() {
+			return true
+		}
+	}
+
+	return false
+}
+
 // negative returns a set of coins with all amount negative.
 //
 // TODO: Remove once unsigned integers are used.

types/coin_test.go

@@ -283,6 +283,20 @@ func (s *coinTestSuite) TestCoinIsZero() {
 	s.Require().False(res)
 }
 
+func (s *coinTestSuite) TestCoinIsNil() {
+	coin := sdk.Coin{}
+	res := coin.IsNil()
+	s.Require().True(res)
+
+	coin = sdk.Coin{Denom: "uatom"}
+	res = coin.IsNil()
+	s.Require().True(res)
+
+	coin = sdk.NewInt64Coin(testDenom1, 1)
+	res = coin.IsNil()
+	s.Require().False(res)
+}
+
 func (s *coinTestSuite) TestFilteredZeroCoins() {
 	cases := []struct {
 		name     string
@@ -945,6 +959,19 @@ func (s *coinTestSuite) TestCoinsIsAnyGT() {
 	}
 }
 
+func (s *coinTestSuite) TestCoinsIsAnyNil() {
+	twoAtom := sdk.NewInt64Coin("atom", 2)
+	fiveAtom := sdk.NewInt64Coin("atom", 5)
+	threeEth := sdk.NewInt64Coin("eth", 3)
+	nilAtom := sdk.Coin{Denom: "atom"}
+
+	s.Require().True(sdk.Coins{twoAtom, fiveAtom, threeEth, nilAtom}.IsAnyNil())
+	s.Require().True(sdk.Coins{twoAtom, nilAtom, fiveAtom, threeEth}.IsAnyNil())
+	s.Require().True(sdk.Coins{nilAtom, twoAtom, fiveAtom, threeEth}.IsAnyNil())
+	s.Require().False(sdk.Coins{twoAtom, fiveAtom, threeEth}.IsAnyNil())
+
+}
+
 func (s *coinTestSuite) TestMarshalJSONCoins() {
 	cdc := codec.NewLegacyAmino()
 	sdk.RegisterLegacyAminoCodec(cdc)

types/tx/types.go

@@ -58,6 +58,13 @@ func (t *Tx) ValidateBasic() error {
 		)
 	}
 
+	if fee.Amount.IsAnyNil() {
+		return sdkerrors.Wrapf(
+			sdkerrors.ErrInsufficientFee,
+			"invalid fee provided: null",
+		)
+	}
+
 	if fee.Amount.IsAnyNegative() {
 		return sdkerrors.Wrapf(
 			sdkerrors.ErrInsufficientFee,