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

server/channels/store/sqlstore/store_test.go

@@ -795,6 +795,50 @@ func TestReplicaLagQuery(t *testing.T) {
 	}
 }
 
+func TestInvalidReplicaLagDataSource(t *testing.T) {
+	logger := mlog.CreateConsoleTestLogger(t)
+
+	testDrivers := []string{
+		model.DatabaseDriverPostgres,
+		model.DatabaseDriverMysql,
+	}
+
+	for _, driver := range testDrivers {
+		t.Run(driver, func(t *testing.T) {
+			settings, err := makeSqlSettings(driver)
+			if err != nil {
+				t.Skip(err)
+			}
+
+			// Set an invalid DataSource that will fail to connect
+			settings.ReplicaLagSettings = []*model.ReplicaLagSettings{{
+				DataSource:       model.NewPointer("invalid://connection/string"),
+				QueryAbsoluteLag: model.NewPointer("SELECT 1"),
+				QueryTimeLag:     model.NewPointer("SELECT 1"),
+			}}
+
+			mockMetrics := &mocks.MetricsInterface{}
+			mockMetrics.On("RegisterDBCollector", mock.AnythingOfType("*sql.DB"), "master")
+
+			store := &SqlStore{
+				rrCounter:   0,
+				srCounter:   0,
+				settings:    settings,
+				metrics:     mockMetrics,
+				logger:      logger,
+				quitMonitor: make(chan struct{}),
+				wgMonitor:   &sync.WaitGroup{},
+			}
+
+			require.NoError(t, store.initConnection())
+			defer store.Close()
+
+			// Verify no replica lag handles were added despite having ReplicaLagSettings
+			assert.Equal(t, 0, len(store.replicaLagHandles))
+		})
+	}
+}
+
 var errDriverMismatch = errors.New("database drivers mismatch")
 var errDriverUnsupported = errors.New("database driver not supported")