Restore typehint but make it nullable

[aaronjorbin]

We can still get the benefit of the type hint by making it nullable instead of removing it all together. This will work down to php7.1 which is below the minimum WP version.

See https://3v4l.org/1Mdrl for the deprecation and https://3v4l.org/o764l for the nullable version

Originally added in https://core.trac.wordpress.org/changeset/59838 Removed in https://core.trac.wordpress.org/changeset/59839

Trac Ticket: https://core.trac.wordpress.org/ticket/62716

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