prevent php8 warning, fixes #2216

Closes #2216

Merge request studip/studip!1447

app/views/calendar/single/_calhead_label_day.php

 <span class="hidden-tiny-down"><?= strftime('%A, ', $atime) ?></span>
 <?= strftime('%d.%m.%Y', $atime) ?>
-<span class="hidden-medium-down" style="font-size: 12pt; color: #bbb; font-weight: bold;"><?= $hd = holiday($atime) ? $hd['name'] : '' ?></span>
+<span class="hidden-medium-down" style="font-size: 12pt; color: #bbb; font-weight: bold;"><? if ($hd = holiday($atime)) echo $hd['name']; ?></span>