fix(group): Allow group total weight to be 0 (#11682)

## Description

Closes: #11651 

I did an audit of all places where we use the group's `TotalWeight`, and made sure it can be 0.



---

### 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/master/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/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/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

*All items are required. Please add a note if the item is not applicable and please add
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)

x/group/keeper/invariants.go

@@ -85,7 +85,7 @@ func GroupTotalWeightInvariantHelper(ctx sdk.Context, key storetypes.StoreKey, g
 			}
 		}
 
-		groupWeight, err := groupmath.NewPositiveDecFromString(groupInfo.GetTotalWeight())
+		groupWeight, err := groupmath.NewNonNegativeDecFromString(groupInfo.GetTotalWeight())
 		if err != nil {
 			msg += fmt.Sprintf("error while parsing non-nengative decimal for group with ID %d\n%v\n", groupInfo.Id, err)
 			return msg, broken

x/group/keeper/keeper_test.go

@@ -2753,7 +2753,7 @@ func (s *TestSuite) TestLeaveGroup() {
 			math.NewDecFromInt64(0),
 		},
 		{
-			"valid testcase: decision policy is not present",
+			"valid testcase: decision policy is not present (and group total weight can be 0)",
 			&group.MsgLeaveGroup{
 				GroupId: groupID2,
 				Address: member1.String(),

x/group/keeper/msg_server.go

@@ -97,9 +97,9 @@ func (k Keeper) CreateGroup(goCtx context.Context, req *group.MsgCreateGroup) (*
 func (k Keeper) UpdateGroupMembers(goCtx context.Context, req *group.MsgUpdateGroupMembers) (*group.MsgUpdateGroupMembersResponse, error) {
 	ctx := sdk.UnwrapSDKContext(goCtx)
 	action := func(g *group.GroupInfo) error {
-		totalWeight, err := math.NewPositiveDecFromString(g.TotalWeight)
+		totalWeight, err := math.NewNonNegativeDecFromString(g.TotalWeight)
 		if err != nil {
-			return err
+			return sdkerrors.Wrap(err, "group total weight")
 		}
 		for i := range req.MemberUpdates {
 			if err := k.assertMetadataLength(req.MemberUpdates[i].Metadata, "group member metadata"); err != nil {
@@ -800,7 +800,7 @@ func (k Keeper) LeaveGroup(goCtx context.Context, req *group.MsgLeaveGroup) (*gr
 		return nil, sdkerrors.Wrap(err, "group")
 	}
 
-	groupWeight, err := math.NewPositiveDecFromString(groupInfo.TotalWeight)
+	groupWeight, err := math.NewNonNegativeDecFromString(groupInfo.TotalWeight)
 	if err != nil {
 		return nil, err
 	}