Gracefully handle error when all pixels are masked out by alpha

If you mask out all the pixels using an alpha mask:

```
poetry run pylette --filename docs/example_imgs/pylette_gui.png --alpha-mask-threshold 255
```
You get an error:
```
ValueError: Found array with 0 sample(s) (shape=(0, 4)) while a minimum of 1 is required by KMeans.
```