Can't delete empty experiment if it's the only experiment

## Issue

Reproduction:

- Start breksta without a pmt.db present
- Start an experiment without an ADC
- Try to delete said experiment

```log
Breksta - WARNING - Experiment 2 not found
Breksta - WARNING - Cannot create preview. Dataframe empty.
Breksta - CRITICAL - Cannot delete data for experiment 2 because there are no readings.
Breksta - WARNING - Database has no data... table is empty
```