[php-src] Issue #21612: htmlentities() returns an empty string

Issue: https://github.com/php/php-src/issues/21612
Author: exakat

### Description

htmlentities() returns empty string when applied on certain characters, with encoding such as big5. 

The following code:

```php
<?php

$string = "éù";
var_dump( htmlentities($string, ENT_QUOTES, "big5"));

$string = "Café & 'quotes' and € euro";
var_dump( htmlentities($string, ENT_QUOTES, "big5"));

```
(also here) : https://3v4l.org/BYPMD#v8.5.3

Resulted in this output:
```
Notice: htmlentities(): Only basic entities substitution is supported for multi-byte encodings other than UTF-8; functionality is equivalent to htmlspecialchars in /in/97KgC on line 4
string(4) "éù"
29
Notice: htmlentities(): Only basic entities substitution is supported for multi-byte encodings other than UTF-8; functionality is equivalent to htmlspecialchars in /in/97KgC on line 8
string(0) ""
```

But I expected this output instead:
```
string(4) "éù"
string(29) "Café & 'quotes' and € euro"
```
With the first string, nothing happens, but the incoming is returned. 
With the second string, something happens, and the whole string is removed. That is excessive.

### PHP Version

```plain
PHP 8.5.4, 
check with versions since 8.2+
```

### Operating System

_No response_