grapheme_extract() returns false instead of empty string when the offset is equal to the length of the haystack

Description

The following code:

<?php
var_dump(grapheme_extract('', 42));
var_dump(grapheme_extract('ab', 42, offset: 2));

Resulted in this output:

bool(false)
bool(false)

But I expected this output instead:

string(0) ""
string(0) ""

Per documentation, the offset ought to be less then or equal to the length of the string (which I consider an appropriate range).

PHP Version

PHP 8.3

Operating System

No response

首先复现报告中的两个 grapheme_extract() 调用，然后在 php-src 中定位该函数的实现及相关测试。验证等于 haystack 长度的 offset 在两种情况下都返回空字符串，同时其他 offset 的行为保持不变。