Fix for the case when the input text is empty

[almmos]

Hello,

I have encountered a specific edge case when the input text is empty.
This is on php 8.2 on FreeBSD 14.1

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in ./vendor/erusev/parsedown/Parsedown.php on line 43

Due to the fact that the input text is immediately used to call to protected function textElements, there will be at least a deprecated warning throw out by str_replace php function.

As a temporary workaround, I have inserted on line 26, as the first line of code in function text($text), the next one-liner snippet and moved anything else down:
if (strlen($text) == 0) return "";

Hope this will help somebody.