fix: WithdrawRewards event emit value when no rewards (#9599)

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## Description

Rewards values of 0 were omitted from the `WithdrawRewards` event.  To provide consistency in event responses, this is caught in the keeper and emitted as a value of  `0[baseDenom]`.

Closes: #8581 

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---

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CHANGELOG.md

@@ -67,6 +67,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 * (types) [\#9627](https://github.com/cosmos/cosmos-sdk/pull/9627) Fix nil pointer panic on `NewBigIntFromInt`
 * (x/genutil) [\#9574](https://github.com/cosmos/cosmos-sdk/pull/9575) Actually use the `gentx` client tx flags (like `--keyring-dir`)
+* (x/distribution) [\#9599](https://github.com/cosmos/cosmos-sdk/pull/9599) Withdraw rewards event now includes a value attribute even if there are 0 rewards (due to situations like 100% commission).
 
 
 ## [v0.43.0-rc0](https://github.com/cosmos/cosmos-sdk/releases/tag/v0.43.0-rc0) - 2021-06-25

x/distribution/keeper/delegation_test.go

@@ -626,3 +626,72 @@ func TestCalculateRewardsMultiDelegatorMultWithdraw(t *testing.T) {
 	// commission should be zero
 	require.True(t, app.DistrKeeper.GetValidatorAccumulatedCommission(ctx, valAddrs[0]).Commission.IsZero())
 }
+
+func Test100PercentCommissionReward(t *testing.T) {
+	app := simapp.Setup(false)
+	ctx := app.BaseApp.NewContext(false, tmproto.Header{})
+
+	tstaking := teststaking.NewHelper(t, ctx, app.StakingKeeper)
+	addr := simapp.AddTestAddrs(app, ctx, 2, sdk.NewInt(1000000000))
+	valAddrs := simapp.ConvertAddrsToValAddrs(addr)
+	initial := int64(20)
+
+	// set module account coins
+	distrAcc := app.DistrKeeper.GetDistributionAccount(ctx)
+	require.NoError(t, testutil.FundModuleAccount(app.BankKeeper, ctx, distrAcc.GetName(), sdk.NewCoins(sdk.NewCoin(sdk.DefaultBondDenom, sdk.NewInt(1000)))))
+	app.AccountKeeper.SetModuleAccount(ctx, distrAcc)
+
+	tokens := sdk.DecCoins{sdk.NewDecCoinFromDec(sdk.DefaultBondDenom, sdk.NewDec(initial))}
+
+	// create validator with 100% commission
+	tstaking.Commission = stakingtypes.NewCommissionRates(sdk.NewDecWithPrec(10, 1), sdk.NewDecWithPrec(10, 1), sdk.NewDec(0))
+	tstaking.CreateValidator(valAddrs[0], valConsPk1, sdk.NewInt(100), true)
+	app.StakingKeeper.Delegation(ctx, sdk.AccAddress(valAddrs[0]), valAddrs[0])
+
+	// end block to bond validator
+	staking.EndBlocker(ctx, app.StakingKeeper)
+	// next block
+	ctx = ctx.WithBlockHeight(ctx.BlockHeight() + 1)
+
+	// fetch validator
+	val := app.StakingKeeper.Validator(ctx, valAddrs[0])
+
+	// allocate some rewards
+	app.DistrKeeper.AllocateTokensToValidator(ctx, val, tokens)
+
+	// end block
+	staking.EndBlocker(ctx, app.StakingKeeper)
+
+	// next block
+	ctx = ctx.WithBlockHeight(ctx.BlockHeight() + 1)
+
+	// allocate some more rewards
+	app.DistrKeeper.AllocateTokensToValidator(ctx, val, tokens)
+
+	// next block
+	ctx = ctx.WithBlockHeight(ctx.BlockHeight() + 1)
+
+	// allocate some more rewards
+	app.DistrKeeper.AllocateTokensToValidator(ctx, val, tokens)
+
+	rewards, err := app.DistrKeeper.WithdrawDelegationRewards(ctx, sdk.AccAddress(valAddrs[0]), valAddrs[0])
+	require.NoError(t, err)
+
+	denom, _ := sdk.GetBaseDenom()
+	zeroRewards := sdk.Coins{
+		sdk.Coin{
+			Denom:  denom,
+			Amount: sdk.ZeroInt(),
+		},
+	}
+	require.True(t, rewards.IsEqual(zeroRewards))
+	events := ctx.EventManager().Events()
+	lastEvent := events[len(events)-1]
+	hasValue := false
+	for _, attr := range lastEvent.Attributes {
+		if string(attr.Key) == "amount" && string(attr.Value) == "0" {
+			hasValue = true
+		}
+	}
+	require.True(t, hasValue)
+}

x/distribution/keeper/keeper.go

@@ -99,6 +99,14 @@ func (k Keeper) WithdrawDelegationRewards(ctx sdk.Context, delAddr sdk.AccAddres
 		return nil, err
 	}
 
+	if rewards.IsZero() {
+		baseDenom, _ := sdk.GetBaseDenom()
+		rewards = sdk.Coins{sdk.Coin{
+			Denom:  baseDenom,
+			Amount: sdk.ZeroInt(),
+		}}
+	}
+
 	ctx.EventManager().EmitEvent(
 		sdk.NewEvent(
 			types.EventTypeWithdrawRewards,