Error when running datenapi example

Example from https://github.com/bundesAPI/marktstammdaten-api/blob/main/python-client/docs/DatenApi.md
```python
import time
from deutschland import marktstammdaten
from deutschland.marktstammdaten.api import daten_api
from deutschland.marktstammdaten.model.einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get200_response import EinheitEinheitJsonGetErweiterteOeffentlicheEinheitStromerzeugungGet200Response
from pprint import pprint
# Defining the host is optional and defaults to https://www.marktstammdatenregister.de/MaStR
# See configuration.py for a list of all supported configuration parameters.
configuration = marktstammdaten.Configuration(
host = "https://www.marktstammdatenregister.de/MaStR"
)

# Enter a context with an instance of the API client
with marktstammdaten.ApiClient() as api_client:
# Create an instance of the API class
api_instance = daten_api.DatenApi(api_client)
sort = "sort_example" # str | Spalte, nach der auf- oder absteigend sortiert werden soll (optional)
page = 1 # int | Seite, die geladen werden soll (optional)
page_size = 1 # int | Anzahl an Einträgen pro Seite (optional)
filter = "filter_example" # str | Syntax: Feld-name~[eq|neq|sw|ct|nct|ew|null|nn]~'Wert'~[and|or]~... (optional)

# example passing only required values which don't have defaults set
# and optional values
try:
# Erweiterte öffentliche Daten zur Stromerzeugung
api_response = api_instance.einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get(sort=sort, page=page, page_size=page_size, filter=filter)
pprint(api_response)
except marktstammdaten.ApiException as e:
print("Exception when calling DatenApi->einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get: %s\n" % e)
```

```
Traceback (most recent call last):
File "main_marktstammdaten.py", line 26, in
api_response = api_instance.einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get(sort=sort, page=page, page_size=page_size, filter=filter)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/api/daten_api.py", line 168, in einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get
return self.einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get_endpoint.call_with_http_info(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/api_client.py", line 997, in call_with_http_info
return self.api_client.call_api(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/api_client.py", line 452, in call_api
return self.__call_api(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/api_client.py", line 249, in __call_api
return_data = self.deserialize(response_data, response_type, _check_type)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/api_client.py", line 357, in deserialize
deserialized_data = validate_and_convert_types(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1611, in validate_and_convert_types
converted_instance = attempt_convert_item(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1485, in attempt_convert_item
raise conversion_exc
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1472, in attempt_convert_item
return deserialize_model(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1377, in deserialize_model
return model_class._new_from_openapi_data(**kw_args)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 49, in wrapped_init
return fn(_self, *args, **kwargs)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 359, in _new_from_openapi_data
return cls._from_openapi_data(*args, **kwargs)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 49, in wrapped_init
return fn(_self, *args, **kwargs)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model/einheit_einheit_json_get_erweiterte_oeffentliche_einheit_stromerzeugung_get200_response.py", line 198, in _from_openapi_data
setattr(self, var_name, var_value)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 180, in __setattr__
self[attr] = value
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 497, in __setitem__
self.set_attribute(name, value)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 156, in set_attribute
value = validate_and_convert_types(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1661, in validate_and_convert_types
input_value[index] = validate_and_convert_types(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1611, in validate_and_convert_types
converted_instance = attempt_convert_item(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1485, in attempt_convert_item
raise conversion_exc
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1472, in attempt_convert_item
return deserialize_model(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1377, in deserialize_model
return model_class._new_from_openapi_data(**kw_args)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 49, in wrapped_init
return fn(_self, *args, **kwargs)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 359, in _new_from_openapi_data
return cls._from_openapi_data(*args, **kwargs)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 49, in wrapped_init
return fn(_self, *args, **kwargs)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model/entry.py", line 629, in _from_openapi_data
setattr(self, var_name, var_value)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 180, in __setattr__
self[attr] = value
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 497, in __setitem__
self.set_attribute(name, value)
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 156, in set_attribute
value = validate_and_convert_types(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1611, in validate_and_convert_types
converted_instance = attempt_convert_item(
File "/home/raphael/.cache/pypoetry/virtualenvs/deutschland-test-pypi-tmto6Anq-py3.8/lib/python3.8/site-packages/deutschland/marktstammdaten/model_utils.py", line 1466, in attempt_convert_item
raise get_type_error(
deutschland.marktstammdaten.exceptions.ApiTypeError: Invalid type for variable 'gemarkung'. Required value type is str and passed type was NoneType at ['received_data']['data'][0]['gemarkung']

```

Start by running the example in docs/DatenApi.md and compare the response handling with the traceback. Inspect the generated response model and model/entry.py, along with model_utils.py, focusing on the null gemarkung value in received_data['data'][0]. The example is done when this response no longer raises ApiTypeError during deserialization.

Written by the indexing model from the issue text.