Cache API: Address code quality issues with wp_cache_switch_to_blog().

This addresses 3 PHPStan errors at rule level 8:

* `arguments.count`: Function `wp_cache_switch_to_blog_fallback()` invoked with 1 parameter, 0 required.
* `method.nonObject`: Cannot call method `switch_to_blog()` on `class-string|object`.                                                             
* `missingType.return`: Function `wp_cache_switch_to_blog()` has no return type specified.                                                                                                                                         

Developed as subset of #11151

Follow-up to r61760.

Props westonruter, johnjamesjacoby.
See #23290, #64238.


git-svn-id: https://develop.svn.wordpress.org/trunk@62028 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/cache-compat.php

@@ -327,11 +327,11 @@ function wp_cache_set_multiple_salted( $data, $group, $salt, $expire = 0 ) {
 	 *
 	 * @param int $blog_id Site ID.
 	 */
-	function wp_cache_switch_to_blog( $blog_id ) {
+	function wp_cache_switch_to_blog( $blog_id ): void {
 		global $wp_object_cache;
 
 		// Attempt to use the drop-in object cache method if it exists.
-		if ( method_exists( $wp_object_cache, 'switch_to_blog' ) ) {
+		if ( is_object( $wp_object_cache ) && method_exists( $wp_object_cache, 'switch_to_blog' ) ) {
 			$wp_object_cache->switch_to_blog( $blog_id );
 			return;
 		}
@@ -340,6 +340,6 @@ function wp_cache_switch_to_blog( $blog_id ) {
 		 * Perform a fallback blog switch, which will reinitialize the caches
 		 * for the new blog ID.
 		 */
-		wp_cache_switch_to_blog_fallback( $blog_id );
+		wp_cache_switch_to_blog_fallback();
 	}
 endif;