Improve error handling in `ds_to_flat`

[robbibt]

Better error handling for empty tiles where no pixels are returned from ds_to_flat function:

2024-09-26 05:32:20 ERROR [1.1.0] [2020] [x152y162]: Failed to run process with error numpy.nanmax raises on a.shape[axis]==0; So Bottleneck too.
2024-09-26 04:57:47 ERROR [1.1.0] [2020] [x081y128]: Failed to run process with error numpy.nanmax raises on a.shape[axis]==0; So Bottleneck too. 
2024-09-26 04:40:39 ERROR [1.1.0] [2020] [x155y105]: Failed to run process with error numpy.nanmax raises on a.shape[axis]==0; So Bottleneck too.