[5.0] Make sure the editor content is a string, not null

[Fedik]

Summary of Changes

Make sure the editor content is a string.

Testing Instructions

Run following, somewhere in layout or in template:

$editor = Joomla\CMS\Editor\Editor::getInstance('tinymce');
echo $editor->display('text', null);

Actual result BEFORE applying this Pull Request

An error

Expected result AFTER applying this Pull Request

You get rendered editor

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org:
• No documentation changes for manual.joomla.org needed

[HLeithner]

thanks