Bring back passing single record support for `Preloader`

I removed redundant `Array.wrap(records)` since `Preloader` is nodoc
class and Active Record always pass `records` as an array to
`Preloader`.
But if users relies on that behavior, it is not worth dropping its
behavior.

Fixes #31661.

activerecord/lib/active_record/associations/preloader.rb

@@ -83,7 +83,7 @@ class Preloader #:nodoc:
       #   { author: :avatar }
       #   [ :books, { author: :avatar } ]
       def preload(records, associations, preload_scope = nil)
-        records = records.compact
+        records = Array.wrap(records).compact
 
         if records.empty?
           []