Editor: Reduce the use of the _wp_array_get() function to improve performance.

`_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.

git-svn-id: https://develop.svn.wordpress.org/trunk@56709 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json-resolver.php

@@ -356,7 +356,7 @@ class WP_Theme_JSON_Resolver {
 
 			if (
 				isset( $block_type->supports['spacing']['blockGap']['__experimentalDefault'] ) &&
-				null === _wp_array_get( $config, array( 'styles', 'blocks', $block_name, 'spacing', 'blockGap' ), null )
+				! isset( $config['styles']['blocks'][ $block_name ]['spacing']['blockGap'] )
 			) {
 				/*
 				 * Ensure an empty placeholder value exists for the block, if it provides a default blockGap value.