Files/FileList::current() - Check files array key

For some unknown reason, doing git precommit hook with husky and
PHP 7.4, FileList method `current` tries to access undefined array
index and throws RuntimeException.

After adding the `isset($this->files[$path]) === false` check,
linting works as expected.

Remove redundant check

src/Files/FileList.php

@@ -184,7 +184,7 @@ public function rewind()
     public function current()
     {
         $path = key($this->files);
-        if ($this->files[$path] === null) {
+        if (isset($this->files[$path]) === false) {
             $this->files[$path] = new LocalFile($path, $this->ruleset, $this->config);
         }