`_wp_array_get()` is an expensive function, and it's called thousands of times on each page view on the front end. While the function performance was slightly improved in #58376, it is still called more times than it should be.
This commit aims to further optimize its usage:
* In many cases, `_wp_array_get()` can be replaced with a much simpler and faster `isset()` check.
* The `isset()` function is capable of checking nested arrays, so `isset( $foo['a']['b']['c'] )` will return false even if `$foo['a']` is unset, without throwing any errors or warnings.
* When `_wp_array_get()` cannot be directly replaced with `isset()`, it would be good practice to wrap it in an `isset()` function so that `_wp_array_get()` only runs when it needs to.
Original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/51116#51116 Performance improvement: Reduce the use of the _wp_array_get() function]
Follow-up to [55851], [56382].
Props aristath, jrf, spacedmonkey, mukesh27, swissspidy, hellofromTonya.
Fixes#59405.
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