General: Remove file_exists() checks after calling realpath().

`realpath()` already checks if the file exists, and returns `false` on failure. The additional `file_exists()` check is not necessary and can be removed, improving the performance.

This commit simplifies the checks in two functions:
* `register_block_type_from_metadata()`
* `wp_json_file_decode()`

Note: In both of these cases, the values are passed through `wp_normalize_path()` after `realpath()`, so if the file does not exist, the `false` value gets converted to an empty string. The updated checks work both for `false` and `''` values.

Though this is a small tweak, it saves a lot of checks since one of the places we do this is when registering block styles, so it runs quite a few times on each page load.

Reference: [https://www.php.net/manual/en/function.realpath.php PHP Manual: realpath()].

Follow-up to [51599], [54132], [54290], [54291].

Props aristath.
Fixes #56654.
Built from https://develop.svn.wordpress.org/trunk@54309


git-svn-id: http://core.svn.wordpress.org/trunk@53868 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/blocks.php

@@ -442,7 +442,7 @@ function register_block_type_from_metadata( $file_or_folder, $args = array() ) {
 				remove_block_asset_path_prefix( $metadata['render'] )
 			)
 		);
-		if ( file_exists( $template_path ) ) {
+		if ( $template_path ) {
 			/**
 			 * Renders the block on the server.
 			 *