Fixes 3.8.0 Regression for User Creation Failure When No Nickname

[timnolte]

All Submissions:

• Have you followed the plugin Contributing guideline?
• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

• Closes user-creation failed if claims does not contain nickname_key (3.8.0 regression) #243.
• Fixes the regression caused by the code changes in 3.8.0 which resulted in not performing proper fallbacks for missing cliams such as the nickname.

How to test the changes in this Pull Request:

1. Setup OIDC plugin with an IDP that doesn't provide a nickname or set the nickname in the configuration to blank.
2. Confirm that user creation is successful with a fallback to using displayname.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

This fix addresses a regression is causes in release 3.8.0 that wouldn't allow the fallback to displayname to be used for the WordPress nickname if a nickname key wasn't provided by the IDP or configured in the settings.