No se puede importar la función api en apim

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

## Describe the bug

**Command Name**
`az apim api import`

**Errors:**
```
must be str, not NoneType
Traceback (most recent call last):
python3.6/site-packages/knack/cli.py, ln 233, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 659, in execute
raise ex
cli/core/commands/__init__.py, ln 722, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 715, in _run_job
six.reraise(*sys.exc_info())
...
cli/core/commands/__init__.py, ln 330, in __call__
return self.handler(*args, **kwargs)
azure/cli/core/__init__.py, ln 786, in default_command_handler
return op(**command_args)
cli/command_modules/apim/custom.py, ln 275, in import_apim_api
raise CLIError("Please provide valid value for specificationFormat: " + specification_format)
TypeError: must be str, not NoneType
```

## 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 apim api import --path {} --resource-group {} --service-name {} --api-id {}`

## Expected Behavior

## Environment Summary
```
Linux-5.4.0-62-generic-x86_64-with-debian-buster-sid
Python 3.6.10
Installer: DEB

azure-cli 2.18.0

```
## Additional Context