0 filled areas #122

@emmanuelmathot There are areas in many Sentinel2 GeoZarrs that lack sensor data (e.g. the southwest corner in https://deploy-preview-17355--ol-site.netlify.app/en/latest/examples/geozarr-stretch.html). These are correctly encoded at low resolutions (r720m, r360m, r120m) using the advertised fill_value, so they appear transparent. But at low resolutions, these values are 0, so they appear as black. This can be seen when you zoom in.

I also noticed that at low resolutions, inner shards that are entirely empty are correctly omitted (saves storage and bandwidth), but at higher resolution these are filled with 0.

Originally posted by @ahocevar in openlayers/openlayers#17355 (comment)