Fix some nullable return types in ext/simplexml

Closes GH-7004

Author: kocsismate

ext/simplexml/simplexml.stub.php

@@ -10,7 +10,7 @@ function simplexml_import_dom(SimpleXMLElement|DOMNode $node, ?string $class_nam
 
 class SimpleXMLElement implements Stringable, Countable, RecursiveIterator
 {
-    /** @return array|false */
+    /** @return array|null|false */
     public function xpath(string $expression) {}
 
     /** @return bool */
@@ -31,18 +31,18 @@ public function getNamespaces(bool $recursive = false) {}
     /** @return array|false */
     public function getDocNamespaces(bool $recursive = false, bool $fromRoot = true) {}
 
-    /** @return SimpleXMLIterator */
+    /** @return SimpleXMLElement|null */
     public function children(?string $namespaceOrPrefix = null, bool $isPrefix = false) {}
 
-    /** @return SimpleXMLIterator */
+    /** @return SimpleXMLElement|null */
     public function attributes(?string $namespaceOrPrefix = null, bool $isPrefix = false) {}
 
     public function __construct(string $data, int $options = 0, bool $dataIsURL = false, string $namespaceOrPrefix = "", bool $isPrefix = false) {}
 
-    /** @return SimpleXMLElement */
+    /** @return SimpleXMLElement|null */
     public function addChild(string $qualifiedName, ?string $value = null, ?string $namespace = null) {}
 
-    /** @return SimpleXMLElement */
+    /** @return SimpleXMLElement|null */
     public function addAttribute(string $qualifiedName, ?string $value = null, ?string $namespace = null) {}
 
     /** @return string */

ext/simplexml/simplexml_arginfo.h

@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: 70ace711365bb5e7df3174d4cfe68fb3df34a105 */
+ * Stub hash: 9d5e78ef2ea4e8bda025f9a9e7f0e07d2e9db702 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_OBJ_TYPE_MASK_EX(arginfo_simplexml_load_file, 0, 1, SimpleXMLElement, MAY_BE_FALSE)
 	ZEND_ARG_TYPE_INFO(0, filename, IS_STRING, 0)