fix(Wrapper): Fixes handling of optional `expires_in` attribute in Access Token

[timnolte]

All Submissions:

• [x] Have you followed the plugin Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

• Fixes #439
• Properly handles expires_in being OPTIONAl according to spec. (See: 3.2.2.5. Successful Authentication Response)

How to test the changes in this Pull Request:

1. Review Unit Tests.

Other information:

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

[github-actions[bot]]

Coverage report for commit: 8386cbf File: clover.xml

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      └─────────────────────────┘
 *Legend:* █ = Current Distribution 

Summary - Lines: 5.15% | Methods: 6.25%

Files	Lines	Methods	Branches
/workspaces/openid-connect-generic/includes
functions.php	-	100.00%	100.00%
openid-connect-generic-client-wrapper.php	4.42%	4.00%	100.00%
openid-connect-generic-client.php	-	-	100.00%
openid-connect-generic-login-form.php	-	-	100.00%
openid-connect-generic-option-logger.php	37.80%	44.44%	100.00%
openid-connect-generic-option-settings.php	47.37%	12.50%	100.00%
openid-connect-generic-settings-page.php	-	-	100.00%
/workspaces/openid-connect-generic
openid-connect-generic.php	1.77%	-	100.00%

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