Cannot list Azure Policy 'policy definitions': Unable to deserialize to object: type, AttributeError: 'str' object has no attribute 'get'

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

I can't list policy definitions.

**Command Name**
`az policy definition list`

**Errors:**
```
The command failed with an unexpected error. Here is the traceback:
Unable to deserialize to object: type, AttributeError: 'str' object has no attribute 'get'
Traceback (most recent call last):
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1428, in _deserialize
found_value = key_extractor(attr, attr_desc, data)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1172, in rest_key_extractor
return working_data.get(key)
AttributeError: 'str' object has no attribute 'get'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/knack/cli.py", line 231, in invoke
cmd_result = self.invocation.execute(args)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/azure/cli/core/commands/__init__.py", line 657, in execute
raise ex
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/azure/cli/core/commands/__init__.py", line 720, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/azure/cli/core/commands/__init__.py", line 704, in _run_job
result = list(result)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/azure/core/paging.py", line 129, in __next__
return next(self._page_iterator)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/azure/core/paging.py", line 84, in __next__
self.continuation_token, self._current_page = self._extract_data(self._response)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/azure/mgmt/resource/policy/v2020_09_01/operations/_policy_definitions_operations.py", line 541, in extract_data
deserialized = self._deserialize('PolicyDefinitionListResult', pipeline_response)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1368, in __call__
return self._deserialize(target_obj, data)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1442, in _deserialize
value = self.deserialize_data(raw_value, attr_desc['type'])
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1618, in deserialize_data
return self.deserialize_type[iter_type](data, data_type[1:-1])
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1649, in deserialize_iter
return [self.deserialize_data(a, iter_type) for a in attr]
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1649, in
return [self.deserialize_data(a, iter_type) for a in attr]
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1631, in deserialize_data
return self._deserialize(obj_type, data)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1442, in _deserialize
value = self.deserialize_data(raw_value, attr_desc['type'])
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1618, in deserialize_data
return self.deserialize_type[iter_type](data, data_type[1:-1])
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1665, in deserialize_dict
return {k: self.deserialize_data(v, dict_type) for k, v in attr.items()}
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1665, in
return {k: self.deserialize_data(v, dict_type) for k, v in attr.items()}
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1631, in deserialize_data
return self._deserialize(obj_type, data)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1442, in _deserialize
value = self.deserialize_data(raw_value, attr_desc['type'])
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1631, in deserialize_data
return self._deserialize(obj_type, data)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1446, in _deserialize
raise_with_traceback(DeserializationError, msg, err)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/exceptions.py", line 51, in raise_with_traceback
raise error.with_traceback(exc_traceback)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1428, in _deserialize
found_value = key_extractor(attr, attr_desc, data)
File "/usr/local/Cellar/azure-cli/2.28.0/libexec/lib/python3.9/site-packages/msrest/serialization.py", line 1172, in rest_key_extractor
return working_data.get(key)
msrest.exceptions.DeserializationError: Unable to deserialize to object: type, AttributeError: 'str' object has no attribute 'get'

```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- `az policy definition list --subscription {}`

## Expected Behavior

A list of policies applied at the subscription scope I specified.

## Environment Summary
```
macOS-11.6-x86_64-i386-64bit, Darwin 20.6.0
Python 3.9.7
Installer: HOMEBREW

azure-cli 2.28.0

Extensions:
aks-preview 0.5.34
azure-devops 0.20.0

```
## Additional Context