bug #19636 [Finder] no PHP warning on empty directory iteration (ggottwald)

This PR was squashed before being merged into the 2.7 branch (closes #19636).

Discussion
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[Finder] no PHP warning on empty directory iteration

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | Since RecursiveDirectoryIterator::SKIP_DOTS is set as flag, opendir gets a warning if an empty directory is reached

Commits
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df314a0 [Finder] no PHP warning on empty directory iteration

Author: fabpot

Iterator/RecursiveDirectoryIterator.php

@@ -137,6 +137,10 @@ public function isRewindable()
             return $this->rewindable;
         }
 
+        if ('' === $this->getPath()) {
+            return $this->rewindable = false;
+        }
+
         if (false !== $stream = @opendir($this->getPath())) {
             $infos = stream_get_meta_data($stream);
             closedir($stream);