Error accessing `X` in `HDF5AnnData` when empty

[lazappi]

If the X slot in an HDF5AnnData is empty, trying to access it gives this error:

Error in rhdf5::h5readAttributes(file, name) : 
  Object /X does not exist in this HDF5 file.

Needs to be handled in some way. Should probably be filled in the same way as Python when creating a new H5AD file.