[CodeQuality] No need to re-update doc when already union null doc on…

… ExplicitReturnNullRector (#5757)

rules-tests/CodeQuality/Rector/ClassMethod/ExplicitReturnNullRector/Fixture/already_union_null_doc.php.inc

@@ -0,0 +1,38 @@
+<?php
+
+namespace Rector\Tests\CodeQuality\Rector\ClassMethod\ExplicitReturnNullRector\Fixture;
+
+class AlreadyUnionNull
+{
+    /**
+     * @return ?string
+     */
+    public function run(int $number)
+    {
+        if ($number > 50) {
+            return 'yes';
+        }
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\CodeQuality\Rector\ClassMethod\ExplicitReturnNullRector\Fixture;
+
+class AlreadyUnionNull
+{
+    /**
+     * @return ?string
+     */
+    public function run(int $number)
+    {
+        if ($number > 50) {
+            return 'yes';
+        }
+        return null;
+    }
+}
+
+?>

rules/CodeQuality/Rector/ClassMethod/ExplicitReturnNullRector.php

@@ -130,14 +130,20 @@ private function transformDocUnionVoidToUnionNull(ClassMethod $classMethod): voi
         }
 
         $newTypes = [];
+        $hasChanged = false;
         foreach ($returnType->getTypes() as $type) {
             if ($type instanceof VoidType) {
                 $type = new NullType();
+                $hasChanged = true;
             }
 
             $newTypes[] = $type;
         }
 
+        if (! $hasChanged) {
+            return;
+        }
+
         $type = $this->typeFactory->createMixedPassedOrUnionTypeAndKeepConstant($newTypes);
         if (! $type instanceof UnionType) {
             return;