Administration: Guard against false transient key in get_cached_events().

Inside `WP_Community_Events::get_cached_events()`, `WP_Community_Events::get_events_transient_key()` is used to retrieve the transient key name, based on the user's location.  However, the transient key can potentially return `false`, resulting in a call to `get_site_transient()` with the `$key` being `false`.

This change first attempts to evaluate and guard against a `false` return from `WP_Community_Events::get_events_transient_key()`.  The result is an early `false` return from `WP_Community_Events::get_cached_events()`.

Props malthert, rafiahmedd, audrasjb, costdev.
Fixes #55888.

git-svn-id: https://develop.svn.wordpress.org/trunk@54338 602fd350-edb4-49c9-b593-d223f7449a82

Author: dream-encode

src/wp-admin/includes/class-wp-community-events.php

@@ -353,8 +353,12 @@ protected function cache_events( $events, $expiration = false ) {
 	 *                     on success, false on failure.
 	 */
 	public function get_cached_events() {
-		$cached_response = get_site_transient( $this->get_events_transient_key( $this->user_location ) );
+		$transient_key = $this->get_events_transient_key( $this->user_location );
+		if ( ! $transient_key ) {
+			return false;
+		}
 
+		$cached_response = get_site_transient( $transient_key );
 		if ( isset( $cached_response['events'] ) ) {
 			$cached_response['events'] = $this->trim_events( $cached_response['events'] );
 		}