fix: emit rewards_withdrawn only when positive amount is withdrawn

[hallazzang]

Description

Currently, rewards_withdraw event is emitted whenever WithdrawRewards is called.
Fix this to only emit event when positive rewards amount is withdrawn.

farming/x/farming/keeper/reward.go

Lines 216 to 236 in 69db071

	if !rewards.IsZero() {
	if !truncatedRewards.IsZero() {
	if err := k.bankKeeper.SendCoins(ctx, k.GetRewardsReservePoolAcc(ctx), farmerAcc, truncatedRewards); err != nil {
	return nil, err
	}
	}
	k.DecreaseOutstandingRewards(ctx, stakingCoinDenom, rewards)
	}
	staking.StartingEpoch = currentEpoch
	k.SetStaking(ctx, stakingCoinDenom, farmerAcc, staking)
	ctx.EventManager().EmitEvents(sdk.Events{
	sdk.NewEvent(
	types.EventTypeRewardsWithdrawn,
	sdk.NewAttribute(types.AttributeKeyFarmer, farmerAcc.String()),
	sdk.NewAttribute(types.AttributeKeyStakingCoinDenom, stakingCoinDenom),
	sdk.NewAttribute(types.AttributeKeyRewardCoins, truncatedRewards.String()),
	),
	})

---

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