Code Modernization: Formatting: Use null coalescing operator instead of `isset()` ternaries.

Developed as a subset of https://github.com/WordPress/wordpress-develop/pull/10654
Initially developed in https://github.com/WordPress/wordpress-develop/pull/4886

Follow-up to [61435], [61434], [61403], [61433], [61432], [61431], [61430], [61429], [61424], [61404], [61403].

Props costdev, westonruter.
See #58874, #63430.


git-svn-id: https://develop.svn.wordpress.org/trunk@61436 602fd350-edb4-49c9-b593-d223f7449a82