[ticket/12852] Make get_url_parts handle get variable with no value

Apart from direct usage of `get_url_parts` method, it is used internally in [/includes/functions_content.php:205](https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/includes/functions_content.php#L205) - passing result to `build_hidden_fields` function which passes that to `_build_hidden_fields` function. That function generates hidden `<input>` elements. Now the important part is, that `<input name="name" type="">` and `<input name="name">` - they both will generate this url: `?name=`. So no additional fix is required.

[PHPBB3-12852](https://tracker.phpbb.com/browse/PHPBB3-12852)