minor #36644 [FrameworkBundle][CacheWarmupCommand] Append files to preload (fancyweb)

This PR was merged into the 5.1-dev branch.

Discussion
----------

[FrameworkBundle][CacheWarmupCommand] Append files to preload

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Files returned by the warmUp() method must be added to the preload file.

I also added checks to avoid empty preload calls in the preload file like this:
```
$classes = [];
Preloader::preload($classes);
```

Commits
-------

a82c7ab4c0 [FrameworkBundle][CacheWarmupCommand] Append files to preload

Author: nicolas-grekas

Kernel.php

@@ -569,7 +569,7 @@ protected function initializeContainer()
         if ($this->container->has('cache_warmer')) {
             $preload = (array) $this->container->get('cache_warmer')->warmUp($this->container->getParameter('kernel.cache_dir'));
 
-            if (method_exists(Preloader::class, 'append') && file_exists($preloadFile = $cacheDir.'/'.$class.'.preload.php')) {
+            if ($preload && method_exists(Preloader::class, 'append') && file_exists($preloadFile = $cacheDir.'/'.$class.'.preload.php')) {
                 Preloader::append($preloadFile, $preload);
             }
         }