Intersect obs fails if AnnData doesn't have X

[grst]

Describe the bug
Running muon.pp.intersect_obs on a MuData object that contains an AnnData object with X=None, it fails

To Reproduce

from anndata import AnnData
import muon as mu
from mudata import MuData
import numpy as np
import pandas as pd

md = MuData({
    "a": AnnData(None, obs=pd.DataFrame(index=["1", "2", "3"])),
    "b": AnnData(np.ones((2,2)), obs=pd.DataFrame(index=["2", "3"]))
})
mu.pp.intersect_obs(md)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~/anaconda3/envs/scirpy_dev/lib/python3.9/site-packages/muon/_core/preproc.py in filter_obs(data, var, func)
    713         try:
--> 714             data._X = data.X[obs_subset, :]
    715         except TypeError:

TypeError: 'NoneType' object is not subscriptable

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
~/tmp/ipykernel_979716/2023157240.py in <cell line: 1>()
----> 1 mu.pp.intersect_obs(md)

~/anaconda3/envs/scirpy_dev/lib/python3.9/site-packages/muon/_core/preproc.py in intersect_obs(mdata)
    627 
    628     for mod in mdata.mod:
--> 629         filter_obs(mdata.mod[mod], common_obs)
    630 
    631     mdata.update_obs()

~/anaconda3/envs/scirpy_dev/lib/python3.9/site-packages/muon/_core/preproc.py in filter_obs(data, var, func)
    714             data._X = data.X[obs_subset, :]
    715         except TypeError:
--> 716             data._X = data.X[np.where(obs_subset)[0], :]
    717             # For some h5py versions, indexing arrays must have integer dtypes
    718             # https://github.com/h5py/h5py/issues/1847

Expected behaviour
Ignore X, intersect obs/obsm/... as usual

System
mudata master
anndata master

Additional context
AnnData objects generated by scirpy do not have a matrix in X, it only uses obs and obsm.