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.

git-svn-id: https://develop.svn.wordpress.org/trunk@54309 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/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.
 			 *

src/wp-includes/functions.php

@@ -4528,7 +4528,8 @@ function wp_check_jsonp_callback( $callback ) {
 function wp_json_file_decode( $filename, $options = array() ) {
 	$result   = null;
 	$filename = wp_normalize_path( realpath( $filename ) );
-	if ( ! file_exists( $filename ) ) {
+
+	if ( ! $filename ) {
 		trigger_error(
 			sprintf(
 				/* translators: %s: Path to the JSON file. */