Improve handling of zero range datashader aggregations #2842
Ensures datashader aggregation on zero-range data is handled correctly. Previously there were some workarounds that did one of two things:
Both were only workarounds and led to unexpected behavior so the operations now return Images with zero sized arrays. This is also related to holoviz/datashader#612 but doesn't require it since the operations are doing all the guarding against the bad cases that datashader doesn't handle correctly.