Fix some more ext/spl return types

Closes GH-7242

Author: kocsismate

ext/spl/spl_iterators.stub.php

@@ -4,13 +4,13 @@
 
 class EmptyIterator implements Iterator
 {
-    /** @return void */
+    /** @return mixed */
     public function current() {}
 
     /** @return void */
     public function next() {}
 
-    /** @return void */
+    /** @return mixed */
     public function key() {}
 
     /** @return bool */
@@ -109,7 +109,7 @@ public function getMaxDepth() {}
 
 interface OuterIterator extends Iterator
 {
-    /** @return Iterator */
+    /** @return Iterator|null */
     public function getInnerIterator();
 }
 
@@ -303,7 +303,7 @@ public function current() {}
     /** @return void */
     public function next() {}
 
-    /** @return int */
+    /** @return int|null */
     public function getIteratorIndex() {}
 
     /** @return ArrayIterator */

ext/spl/spl_iterators_arginfo.h

@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: da35c3e48b3a09f5f013fdd760ddab6bdfb37967 */
+ * Stub hash: a56f02ad7b9578713f0d37b2cf3d95853a4ea45e */
 
 ZEND_BEGIN_ARG_INFO_EX(arginfo_class_EmptyIterator_current, 0, 0, 0)
 ZEND_END_ARG_INFO()