'NoneType' is not iterable thrown in agent.py get_managed_identity

Seen in the instance view of a VM:

```
VM has reported a failure when processing extension 'AzureMonitorLinuxAgent' (publisher 'Microsoft.Azure.Monitor' and type 'AzureMonitorLinuxAgent'). Error message: "Enable failed with error: argument of type 'NoneType' is not iterable
Stacktrace: Traceback (most recent call last):
File "./agent.py", line 195, in main
exit_code, output = operations[operation]()
File "./agent.py", line 482, in enable
handle_mcs_config(public_settings, protected_settings, default_configs)
File "./agent.py", line 702, in handle_mcs_config
identifier_name, identifier_value, error_msg = get_managed_identity()
File "./agent.py", line 819, in get_managed_identity
if public_settings is not None and "authentication" in public_settings and "managedIdentity" in public_settings.get("authentication"):
TypeError: argument of type 'NoneType' is not iterable
". More information on troubleshooting is available at https://aka.ms/vmextensionlinuxtroubleshoot.
```

Doesn't seem new, just that public_settings needs a null check and perhaps some logging.

Start in agent.py at get_managed_identity, especially the line 819 path shown in the traceback, and trace how public_settings and authentication are populated by handle_mcs_config. Verify the missing or null authentication case and confirm that enabling the extension no longer raises this TypeError; investigate the requested logging only if the existing code and issue context clarify its expected scope.

Written by the indexing model from the issue text.