Raise Error message on manualControls thresholds if group is empty

[Rumengol]

If a group is empty, we need to raise an error message. We are able to get the number of cells within each group in the Groups' size section

```{r groupsSizeMito}
mito_groups <- sort(unique(cplt$mito_groups))
table(cplt@meta.data$mito_groups)
``