'NoneType' object has no attribute 'lower'

## Describe the bug

Got the following error while using the az sql db update command

**Command Name**
`az sql db update`

**Errors:**
```
The command failed with an unexpected error. Here is the traceback:
'NoneType' object has no attribute 'lower'
Traceback (most recent call last):
File "/opt/az/lib/python3.6/site-packages/knack/cli.py", line 231, in invoke
cmd_result = self.invocation.execute(args)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 657, in execute
raise ex
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 720, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 699, in _run_job
result = transform_op(result)
File "/opt/az/lib/python3.6/site-packages/azure/cli/command_modules/sql/_format.py", line 56, in __call__
return self._transform_func(result)
File "/opt/az/lib/python3.6/site-packages/azure/cli/command_modules/sql/_format.py", line 156, in db_transform
result.backupStorageRedundancy = _get_external_backup_storage_redundancy(result.storage_account_type)
File "/opt/az/lib/python3.6/site-packages/azure/cli/command_modules/sql/_format.py", line 406, in _get_external_backup_storage_redundancy
}.get(self.lower(), 'Invalid')
AttributeError: 'NoneType' object has no attribute 'lower'

```

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

- `az sql db update -g {} -n {} -s {} --ha-replicas {} --debug`

## Expected Behavior

## Environment Summary
```
Linux-5.4.72-microsoft-standard-WSL2-x86_64-with-debian-bullseye-sid
Python 3.6.10
Installer: DEB

azure-cli 2.24.0 *

Extensions:
interactive 0.4.4
db-up 0.2.2

```
## Additional Context