[Datasets] Preserve Dataset type in the face of empty block lists and empty blocks.

[clarkzinzow]

When Datasets transformations result in empty block lists or empty blocks, the block (and therefore Dataset) type is dropped or incorrectly set, respectively, leading to downstream operations being unable to switch on the Dataset type in the former case, and such transformations having to manually preserve the block type in the latter case. We should try to preserve the Dataset type in general for both of these cases.