Code Modernization: Use null coalescing operator instead of isset() ternaries in remaining core files.
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 [61456], [61455], [61454], [61453], [61445], [61444], [61443], [61442], [61436], [61435], [61434], [61403], [61433], [61432], [61431], [61430], [61429], [61424], [61404], [61403].
Props costdev, westonruter, jrf, SergeyBiryukov, swissspidy, hellofromTonya, marybaum, oglekler, dmsnell, chaion07, noisysocks, mukesh27.
See #63430.
Fixes #58874.
Built from https://develop.svn.wordpress.org/trunk@61457
git-svn-id: http://core.svn.wordpress.org/trunk@60769 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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