MM-62964: Skip setting replica lag handle in case of an error (#30498)

Previously, we would set it to nil pointer which would eventually
cause a panic when ReplicaLagAbs/ReplicaLagTime would get called.

https://mattermost.atlassian.net/browse/MM-62964

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/sqlstore/store.go

@@ -341,16 +341,17 @@ func (ss *SqlStore) initConnection() error {
 	}
 
 	if len(ss.settings.ReplicaLagSettings) > 0 {
-		ss.replicaLagHandles = make([]*dbsql.DB, len(ss.settings.ReplicaLagSettings))
+		ss.replicaLagHandles = make([]*dbsql.DB, 0, len(ss.settings.ReplicaLagSettings))
 		for i, src := range ss.settings.ReplicaLagSettings {
 			if src.DataSource == nil {
 				continue
 			}
-			ss.replicaLagHandles[i], err = sqlUtils.SetupConnection(ss.Logger(), fmt.Sprintf(replicaLagPrefix+"-%d", i), *src.DataSource, ss.settings, DBReplicaPingAttempts)
+			replicaLagHandle, err := sqlUtils.SetupConnection(ss.Logger(), fmt.Sprintf(replicaLagPrefix+"-%d", i), *src.DataSource, ss.settings, DBReplicaPingAttempts)
 			if err != nil {
 				mlog.Warn("Failed to setup replica lag handle. Skipping..", mlog.String("db", fmt.Sprintf(replicaLagPrefix+"-%d", i)), mlog.Err(err))
 				continue
 			}
+			ss.replicaLagHandles = append(ss.replicaLagHandles, replicaLagHandle)
 		}
 	}
 	return nil