[12.x] Fix translation FileLoader overrides with a missing key

[fabio-ivona]

This PR will fix the translations FileLoader behaviour when multiple overrides are set up and the last one is missing a translation key:

as in Fileloader → loadNamespaceOverrides() the $lines variable is passed to the function through use, its value is used every time in array_replace_recursive(), instead of using the updated $output variable coming from previous iterations of the reduce function

this will cause the last iteration to remove all overrides made in previous ones

I added a failing test for this issue and implemented the fix:

protected function loadNamespaceOverrides(array $lines, $locale, $group, $namespace)
    {
        return (new Collection($this->paths))
-            ->reduce(function ($output, $path) use ($lines, $locale, $group, $namespace) {
+           ->reduce(function ($output, $path) use ($locale, $group, $namespace) {
                $file = "{$path}/vendor/{$namespace}/{$locale}/{$group}.php";

                if ($this->files->exists($file)) {
-                    $lines = array_replace_recursive($lines, $this->files->getRequire($file));
+                    $output = array_replace_recursive($output, $this->files->getRequire($file));
                }

-                return $lines;
+                return $output;
-            }, []);
+            }, $lines);
    }