minor #12206 [Config] fix filelocator with empty name (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[Config] fix filelocator with empty name

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

fix filelocator with empty name + phpdoc in config component

Commits
-------

63b8c07 [DependencyInjection] use inheritdoc for loaders
ddd2fe2 [Config] fix filelocator with empty name

Author: fabpot

Config/FileLocator.php

@@ -47,7 +47,7 @@ public function __construct(KernelInterface $kernel, $path = null, array $paths
      */
     public function locate($file, $currentPath = null, $first = true)
     {
-        if ('@' === $file[0]) {
+        if (isset($file[0]) && '@' === $file[0]) {
             return $this->kernel->locateResource($file, $this->path, $first);
         }