Incorrect stub for RecursiveIterator, return value of getChildren() should be nullable

[sad-spirit]

The docs for RecursiveIterator and stubs in PHP source specify the return value of getChildren() as ?RecursiveIterator, while psalm complains in case of null return value:

https://psalm.dev/r/602a63575d

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/602a63575d

<?php
class Foo {}

class ManyFoos extends Foo
{
    /** @var Foo[] */
    public array $children = [];
}

/**
 * @extends RecursiveArrayIterator<array-key, Foo>
 */
class RecursiveFoos extends RecursiveArrayIterator
{
    public function hasChildren(): bool
    {
        return $this->current() instanceof ManyFoos;
    }
    
    public function getChildren(): ?self
    {
        $current = $this->current();
        return $current instanceof ManyFoos ? new self($current->children) : null;
    }
}

Psalm output (using commit 147505c):

ERROR: NullableReturnStatement - 23:16 - The declared return type 'RecursiveFoos' for RecursiveFoos::getChildren is not nullable, but the function returns 'RecursiveFoos|null'

ERROR: InvalidNullableReturnType - 20:36 - The declared return type 'RecursiveFoos' for RecursiveFoos::getChildren is not nullable, but 'RecursiveFoos|null' contains null

[weirdan]

Appears to be fixed in #10340