Azure Function managed identity in corrupted state

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

## Describe the bug
After a failed deployment using AzureDevOps/deploy via az functionapp a staging slot has had its managed identity corrupted. Trying to show/disable it/re-create fails through the CLI.

**Command Name**
`az functionapp identity show`

**Errors:**
```
'NoneType' object has no attribute 'identity'
Traceback (most recent call last):
Local\Temp\pip-unpacked-wheel-tuw8_17y\knack\cli.py, ln 215, in invoke
azure\cli\core\commands\__init__.py, ln 654, in execute
azure\cli\core\commands\__init__.py, ln 718, in _run_jobs_serially
azure\cli\core\commands\__init__.py, ln 711, in _run_job
AppData\Local\Temp\pip-unpacked-wheel-cpn0gwyu\six.py, ln 703, in reraise
azure\cli\core\commands\__init__.py, ln 688, in _run_job
azure\cli\core\commands\__init__.py, ln 325, in __call__
azure\cli\core\commands\arm.py, ln 756, in handler
azure\cli\core\commands\arm.py, ln 772, in show_exception_handler
azure\cli\core\commands\arm.py, ln 754, in handler
azure\cli\command_modules\appservice\custom.py, ln 773, in show_identity
AttributeError: 'NoneType' object has no attribute 'identity'
```

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

- _Put any pre-requisite steps here..._
- `az functionapp identity show -g {} -n {}`

## Expected Behavior

## Environment Summary
```
Windows-10-10.0.18362-SP0
Python 3.6.8
Installer: MSI

azure-cli 2.12.1 *

```
## Additional Context