Emission costs output doesn't work

I get this error when I use the `emission_costs` output:

```
File "C:\Users\tbland\Documents\Code\MUSE_OS\src\muse\outputs\mca.py", line 871, in sector_emission_costs
i = (np.where(envs))[0][0]
~~~~~~~~~~~~~~~~~~~^^^
IndexError: index 0 is out of bounds for axis 0 with size 0
```

This happens both with MUSE 1.1.0 and the dev version

Relates to the `sector_emission_costs` function in `outputs/mca.py`