What I don't understand is that /dokuwiki/inc/Menu/Item/Recent.php does seem to assign an existing image:
$this->svg = DOKU_INC . 'lib/images/menu/calendar-clock.svg';
And the structure of the code seems consistent with what's in /dokuwiki/inc/Menu/Item/Admin.php.
Yet while the former produces no image, the latter does.
Obviously I'm missing something.
Hopefully someone can fill in the knowledge gap or point me at an explanatory document that I've somehow missed.