Windows System Linux - az aro create fails

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

## Describe the bug

**Command Name**
`az aro create`

**Errors:**
```
'NoneType' object has no attribute 'object_id'
Traceback (most recent call last):
python3.6/site-packages/knack/cli.py, ln 215, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 654, in execute
raise ex
cli/core/commands/__init__.py, ln 718, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 711, in _run_job
six.reraise(*sys.exc_info())
...
cli/core/commands/__init__.py, ln 325, in __call__
return self.handler(*args, **kwargs)
azure/cli/core/__init__.py, ln 776, in default_command_handler
return op(**command_args)
cli/command_modules/aro/custom.py, ln 64, in aro_create
assign_contributor_to_vnet(cmd.cli_ctx, vnet, rp_client_sp.object_id, 'rp_client')
AttributeError: 'NoneType' object has no attribute 'object_id'
```

## 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 aro create --resource-group {} --name {} --vnet {} --master-subnet {} --worker-subnet {} --pull-secret {}`

## Expected Behavior

## Environment Summary
```
Linux-4.19.104-microsoft-standard-x86_64-with-debian-buster-sid
Python 3.6.10
Installer: DEB

azure-cli 2.9.1

```
## Additional Context