Custom CSS: fix a PHP notice (#6154)

* Custom CSS: fix a PHP notice

The `$post` parameter for the `_wp_post_revision_fields` filter doesn’t
necessarily have any of the fields set that this function requires.

If they’re not set, bail early.

* Tweak the check for coding standards and readability

modules/custom-css/custom-css-4.7.php

@@ -149,6 +149,11 @@ public static function customizer_redirect() {
 	 * @return array $fields Modified array to include post_content_filtered.
 	 */
 	public static function _wp_post_revision_fields( $fields, $post ) {
+		// None of the fields in $post are required to be passed in this filter.
+		if ( ! isset( $post['post_type'], $post['ID'] ) ) {
+			return $fields;
+		}
+
 		// If we're passed in a revision, go get the main post instead.
 		if ( 'revision' === $post['post_type'] ) {
 			$main_post_id = wp_is_post_revision( $post['ID'] );