fix: classify client states without consensus states as expired (#941)

## Description

closes: #850

---

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checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

- [ ] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md).
- [ ] Wrote unit and integration [tests](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#testing)
- [ ] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
- [ ] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
- [ ] Added a relevant changelog entry to the `Unreleased` section in `CHANGELOG.md`
- [ ] Re-reviewed `Files changed` in the Github PR explorer
- [ ] Review `Codecov Report` in the comment section below once CI passes

(cherry picked from commit d48f576)

modules/core/02-client/keeper/grpc_query_test.go

@@ -452,7 +452,7 @@ func (suite *KeeperTestSuite) TestQueryClientStatus() {
 					ClientId: path.EndpointA.ClientID,
 				}
 			},
-			true, exported.Unknown.String(),
+			true, exported.Expired.String(),
 		},
 		{
 			"Frozen client status",

modules/light-clients/07-tendermint/types/client_state.go

@@ -78,7 +78,9 @@ func (cs ClientState) Status(
 	// get latest consensus state from clientStore to check for expiry
 	consState, err := GetConsensusState(clientStore, cdc, cs.GetLatestHeight())
 	if err != nil {
-		return exported.Unknown
+		// if the client state does not have an associated consensus state for its latest height
+		// then it must be expired
+		return exported.Expired
 	}
 
 	if cs.IsExpired(consState.Timestamp, ctx.BlockTime()) {

modules/light-clients/07-tendermint/types/client_state_test.go

@@ -47,10 +47,10 @@ func (suite *TendermintTestSuite) TestStatus() {
 			clientState.FrozenHeight = clienttypes.NewHeight(0, 1)
 			path.EndpointA.SetClientState(clientState)
 		}, exported.Frozen},
-		{"client status is unknown", func() {
+		{"client status without consensus state", func() {
 			clientState.LatestHeight = clientState.LatestHeight.Increment().(clienttypes.Height)
 			path.EndpointA.SetClientState(clientState)
-		}, exported.Unknown},
+		}, exported.Expired},
 		{"client status is expired", func() {
 			suite.coordinator.IncrementTimeBy(clientState.TrustingPeriod)
 		}, exported.Expired},