Prevent ErrorException if $parts is empty

If `$level` reaches 0, `array_slice($parts, 0, $level)` returns an empty array. This will issue a "undefined index notice" in the next recursion on `$part = $parts[$level]`. This notice gets transformed to an ErrorException which will cancel further analysis.

src/Index/Index.php

@@ -423,6 +423,10 @@ class Index implements ReadableIndex, \Serializable
      */
     private function removeIndexedDefinition(int $level, array $parts, array &$storage, array &$rootStorage)
     {
+        if (empty($parts)) {
+            return;
+        }
+        
         $part = $parts[$level];
 
         if ($level + 1 === count($parts)) {