HDF5: empty groups and keys
Hi,

With some of the hdf5 files I have, `pandas.HDFStore.groups()` returns an empty list. (as does `.keys()` which iterates over the groups). However, the data are accessible via `.get()` or `.get_node()`.

This is related to #21543 and #21372 where the `.groups()` logic was changed, in particular using `self._handle.walk_groups()` instead of `self._handle.walk_nodes()`, now to be found here:
https://github.com/pandas-dev/pandas/blob/ea2e26ae7d700d7fd363ea5bfc05d2fe3fb8a5ee/pandas/io/pytables.py#L1212


#### Current Output

```python
>>> hdf.groups()
[]
```
```python
>>> hdf.keys()
[]
```

#### Expected Ouptut

List of groups and keys as visible with e.g. `h5dump`.
**Note:** Changing the aforementioned line back to use `.walk_nodes()` fixes the issue and lists the groups and keys properly:

```python
>>> hdf.groups()
[/Data/Table Layout (Table(69462,), zlib(4)) ''
   description := {
...
/Data/Array Layout/2D Parameters/Data Parameters (Table(15,)) ''
   description := {
   "mnemonic": StringCol(itemsize=8, shape=(), dflt=b'', pos=0),
   "description": StringCol(itemsize=48, shape=(), dflt=b'', pos=1),
   "isError": Int64Col(shape=(), dflt=0, pos=2),
   "units": StringCol(itemsize=7, shape=(), dflt=b'', pos=3),
   "category": StringCol(itemsize=31, shape=(), dflt=b'', pos=4)}
   byteorder := 'little'
   chunkshape := (642,)]]
```
```python
>>> hdf.keys()
['/Data/Table Layout',
 '/Metadata/Data Parameters',
 '/Metadata/Experiment Notes',
 '/Metadata/Experiment Parameters',
 '/Metadata/Independent Spatial Parameters',
 '/Metadata/_record_layout',
 '/Data/Array Layout/Layout Description',
 '/Data/Array Layout/1D Parameters/Data Parameters',
 '/Data/Array Layout/2D Parameters/Data Parameters']
```

#### Fix

One solution would be (I guess) to revert #21543, another to fix at least `.keys()` to use `._handle.walk_nodes()` instead of `.groups()` in
https://github.com/pandas-dev/pandas/blob/ea2e26ae7d700d7fd363ea5bfc05d2fe3fb8a5ee/pandas/io/pytables.py#L562

Could also be that it is a bug in `pytables`.

#### Problem background

I was trying to figure out why some hdf5 files open fine with `pandas` but fail with `dask`.
The reason is that `dask` allows wildcards and iterates over the keys to find valid ones. If `.keys()` is empty, reading the files with `dask` fails.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-957.27.2.el7.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.1.post20191125
Cython           : None
pytest           : 5.0.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.2
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.10.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.2
numexpr          : 2.7.0
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.2
sqlalchemy       : None
tables           : 3.6.1
xarray           : 0.14.1
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
