return NULL if path does not exist

[rcannood]

Without this, we get an error when reading an empty h5ad.

Example

import anndata as ad

adata = ad.AnnData(
  shape=(10, 20)
)

adata.write_h5ad("empty.h5ad")

devtools::load_all()
adata <- read_h5ad("empty.h5ad")

Error in `[[.H5File`(file, name) : 
  An object with name X does not exist in this group

Testing

This situation will be covered by #193 by removing X = ... from the dummy_anndata generate_dataset call

[lazappi]

How much have you tested this? I wonder if returning NULL could break some things, like do we use X to get n_vars and n_obs?

[rcannood]

Since X might not always be in the hdf5 file (as in, the group does not even exist), we should use it to derive values like n_obs and n_vars.

Just noticed that I made a mistake when committing my code, hopefully fixed now.

[lazappi]

I checked and it's obs/var that have to exist so I think this should be fine