Uploading pfx with 2 public certificates goes wrong

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

## Describe the bug

**Command Name**
`az functionapp config ssl upload`

**Errors:**
```
'NoneType' object has no attribute 'digest'
Traceback (most recent call last):
Temp\pip-install-yh2ypeu1\knack\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 709, in _run_job
azure\cli\command_modules\appservice\commands.py, ln 55, in _polish_bad_errors
azure\cli\core\commands\__init__.py, ln 688, in _run_job
azure\cli\core\commands\__init__.py, ln 325, in __call__
site-packages\azure\cli\core\__init__.py, ln 776, in default_command_handler
azure\cli\command_modules\appservice\custom.py, ln 2207, in upload_ssl_cert
azure\cli\command_modules\appservice\custom.py, ln 2224, in _get_cert
AttributeError: 'NoneType' object has no attribute 'digest'
```

## 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 config ssl upload --certificate-file {} --certificate-password {} --name {} --resource-group {} --query thumbprint --output tsv`

## Expected Behavior

## Environment Summary
```
Windows-10-10.0.19041-SP0
Python 3.6.6
Installer: MSI

azure-cli 2.9.1 *

```
## Additional Context

Start in azure/cli/command_modules/appservice/custom.py at upload_ssl_cert and _get_cert, using the reported traceback as the entry point. Reproduce the az functionapp config ssl upload command with a PFX containing two public certificates, then verify it completes without the NoneType digest error and returns the requested thumbprint.

Written by the indexing model from the issue text.