Tasmota: tolerate missing power information in StatusSNS (evcc-io#6050)

Co-authored-by: Markus Thierolf <m.thierolf@googlemail.com>

meter/tasmota/types.go

@@ -96,6 +96,9 @@ type StatusSNSResponse struct {
 type Channels []float64
 
 func (ch *Channels) Channel(channel int) (float64, error) {
+	if *ch == nil {
+		return 0, nil
+	}
 	if channel < 1 || channel > len(*ch) {
 		return 0, fmt.Errorf("invalid channel: %d", channel)
 	}