ComparerCollection.sel(), time arg errors when selecting a month where some comparers have no temporal overlap

Steps to reproduce:

This works:
```python
import modelskill as ms
cc = ms.data.oresund()

cc.sel(time=slice("2022-01-03 00:00:00", "2022-01-03 00:00:00")
```

This fails:
```python
import modelskill as ms
cc = ms.data.oresund()

cc.sel(time=slice("2022-01-03")
```

Likely caused by this comparer failing:
```python
cc["Koege"].sel(time=slice("2022-01-03")
```

Expected behaviour:

Should work the same way when using an explicit slice as using a implicit time period. Selecting a time period with no data should return an empty comparer, and on a comparer collection it gets removed (as it already works today).