Fix PHP8.1 Deprecation notices passing null

[afragen]

While testing a feature that occurs during an auto-update, in wp-cron, I found PHP deprecation notices that only occur in PHP8.1. What is happening is null is being passed to certain functions who expect to see a string.

Simply type casting the variable fixes the issue.

In testing Rollback during an auto-update I discovered some PHP Deprecation notices that only occur in PHP8.1 as null is the passed value. Both of these values should be strings. These are easily fixed by type casting.

Trac ticket: https://core.trac.wordpress.org/ticket/57605#ticket

[ocean90]

Duplicate of #56850 and #56434.