SimplePie fix PHP8 uncatched error (#3547)
SimplePie fix PHP8 uncatched error (#3547)

#fix https://github.com/FreshRSS/FreshRSS/pull/3546
When `loadHTML()` is given a null or empty string.

```
PHP Fatal error:  Uncaught ValueError: DOMDocument::loadHTML(): Argument #1 ($source) must not be empty in /var/www/freshrss/lib/SimplePie/SimplePie/Locator.php:83
```
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