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 https://github.com/WordPress/wordpress-develop/pull/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