Removed default None file in Azure AD authenticator (#807)

## Description
<!-- Mandatory: Provide a clear, concise description of the changes and
their purpose -->
- What is being changed?
The default value for the config item was set to None. This caused a
null value to show up in the UI, when then saving it get the string
"null" and save that into the database.

- Why is this change needed?
We end up setting this field to "null" on change. 

- How does this change address the issue?
We no longer send the UI the "None" value and thus the UI won't send us
the "null" value back.

## Type of Change
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- [X] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Documentation update
- [ ] Test update
- [ ] Refactoring (no functional changes)
- [ ] Development environment change
- [ ] Configuration change

## Self-Review Checklist
<!-- These items help ensure quality - they complement our automated CI
checks -->
- [X] I have performed a self-review of my code
- [X] I have added relevant comments to complex code sections
- [X] I have updated documentation where needed
- [x] I have considered the security impact of these changes
- [x] I have considered performance implications
- [x] I have thought about error handling and edge cases
- [x] I have tested the changes in my local environment

## Testing Instructions
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<!-- Must be detailed enough for reviewers to reproduce -->
### Prerequisites
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### Steps to Test
1. Create an azure ad authenticator with junk it in. Note that the
default value for the Username is "null". Save the authenticator. Look
at the authenticator in the API it will have a string value of "null".
2. Apply the patch and everything will be good.
3. 

### Expected Results
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## Additional Context
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### Required Actions
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- [ ] Requires documentation updates
  <!-- API docs, feature docs, deployment guides -->
- [ ] Requires downstream repository changes
  <!-- Specify repos: django-ansible-base, eda-server, etc. -->
- [ ] Requires infrastructure/deployment changes
  <!-- CI/CD, installer updates, new services -->
- [ ] Requires coordination with other teams
  <!-- UI team, platform services, infrastructure -->
- [ ] Blocked by PR/MR: #XXX
  <!-- Reference blocking PRs/MRs with brief context -->

### Screenshots/Logs
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Author: john-westcott-iv

ansible_base/authentication/authenticator_plugins/azuread.py

@@ -49,7 +49,6 @@ class AzureADConfiguration(BaseAuthenticatorConfiguration):
         help_text=_("The name of the field from the assertion to use as the username. If not set will default to name"),
         required=False,
         allow_null=True,
-        default=None,
         ui_field_label=_("Field to use as username"),
     )