Explicitly allow setting a null expiration date via pmpro_set_expiration_date().

[ZebulanStanphill]

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Changes proposed in this Pull Request:

pmpro_set_expiration_date() already works with null values (since $wpdb->update() already works with them), but the PHPDocs do not reflect this. Seeing as that the alternative of zero-dates is not recommended, I think it would be good to officially support passing null to the enddate parameter of this function.

How to test the changes in this Pull Request:

There's not really anything to test, functionality-wise. This just makes static analysis tools like PHPStan/Psalm stop complaining when you pass null as the enddate argument in pmpro_set_expiration_date().

Other information:

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

Changelog entry

Officially support passing null as enddate argument in pmpro_set_expiration_date().