Script Loader: Return early in _wp_theme_json_webfonts_handler if theme.json not present.

Check to see if the theme.json file exists before processing the rest of the code in `_wp_theme_json_webfonts_handler`. This improves performance by not trying to parse the theme.json, early in the bootstrap process. 

Props spacedmonkey, hellofromTonya, flixos90.
Fixes #57814.
Built from https://develop.svn.wordpress.org/trunk@55612


git-svn-id: http://core.svn.wordpress.org/trunk@55124 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/script-loader.php

@@ -3225,6 +3225,10 @@ function _wp_theme_json_webfonts_handler() {
 		return;
 	}
 
+	if ( ! wp_theme_has_theme_json() ) {
+		return;
+	}
+
 	// Webfonts to be processed.
 	$registered_webfonts = array();