nasa ads throws error with example code

Hi, as the title says the nasa_ads query throws an error with the example code:

```python
results = ads.ADS.query_simple('^Persson Origin of water around deeply embedded low-mass protostars')
```

results in

```python
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
Cell In[28], line 1
----> 1 results = ads.ADS.query_simple('^Persson Origin of water around deeply embedded low-mass protostars')

File ~/opt/anaconda3/lib/python3.8/site-packages/astroquery/utils/class_or_instance.py:25, in class_or_instance.__get__..f(*args, **kwds)
23 def f(*args, **kwds):
24 if obj is not None:
---> 25 return self.fn(obj, *args, **kwds)
26 else:
27 return self.fn(cls, *args, **kwds)

File ~/opt/anaconda3/lib/python3.8/site-packages/astroquery/nasa_ads/core.py:70, in ADSClass.query_simple(self, query_string, get_query_payload, get_raw_response, cache)
68 return response
69 # parse the XML response into AstroPy Table
---> 70 resulttable = self._parse_response(response.json())
72 return resulttable

File ~/opt/anaconda3/lib/python3.8/site-packages/astroquery/nasa_ads/core.py:77, in ADSClass._parse_response(self, response)
74 def _parse_response(self, response):
76 try:
---> 77 response['response']['docs'][0]['bibcode']
78 except IndexError:
79 raise RuntimeError('No results returned!')

KeyError: 'bibcode'
```

Start in astroquery/nasa_ads/core.py at ADSClass.query_simple and _parse_response, then reproduce the documented query to inspect the returned response shape. Determine how a response whose first document lacks bibcode should be handled, and verify that the example no longer raises an unexpected KeyError while preserving the intended no-results behavior.

Written by the indexing model from the issue text.