For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
case 'star-empty':
$iconParts[$key] = 'icon-star-0';
break;
$opts->starIconEmpty = FabrikHelperHTML::icon("icon-star-empty", '', '', true);
In any case, I checked the font-awesome css of the template and it had the star-empty class