Fix Scroll:current() annotation

Calling Scroll->current() on an invalid Iterator should be avoided: the Iterator documentation
states that code should first call "valid()" before accessing the "current()" value. Not
doing so can lead to unexpected results.

Scroll will throw an InvalidException when trying to call "current()" on an invalid state.

lib/Elastica/Scroll.php

@@ -2,6 +2,8 @@
 
 namespace Elastica;
 
+use Elastica\Exception\InvalidException;
+
 /**
  * Scroll Iterator.
  *
@@ -43,9 +45,6 @@ class Scroll implements \Iterator
     private $totalPages = 0;
     private $currentPage = 0;
 
-    /**
-     * Constructor.
-     */
     public function __construct(Search $search, string $expiryTime = '1m')
     {
         $this->_search = $search;
@@ -57,8 +56,12 @@ public function __construct(Search $search, string $expiryTime = '1m')
      *
      * @see http://php.net/manual/en/iterator.current.php
      */
-    public function current(): ?ResultSet
+    public function current(): ResultSet
     {
+        if (!$this->_currentResultSet) {
+            throw new InvalidException('Could not fetch the current ResultSet from an invalid iterator. Did you forget to call "valid()"?');
+        }
+
         return $this->_currentResultSet;
     }
 
@@ -67,7 +70,7 @@ public function current(): ?ResultSet
      *
      * @see http://php.net/manual/en/iterator.next.php
      */
-    public function next()
+    public function next(): void
     {
         $this->_currentResultSet = null;
         if ($this->currentPage < $this->totalPages) {