Fix: use explicit returns in WP_Site_Health_Auto_Updates::test_*() methods

[justlevine]

Trac ticket: https://core.trac.wordpress.org/ticket/52217

This PR fixes several instances of WP_Site_Health_Auto_Updates::test_*() potentially returning void instead of their documented return types.

Since these methods are public for some reason, null is used to represent a passed test for backwards-compatibility with the coersion of the previously-returned voids. Previous usage of false is preserved.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

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[apermo]

See comment.

From a technical point of view all good, just the doc thing feels wrong.

[SergeyBiryukov]

Thanks for the PR! Merged in r59340.