Fix creating events with old (< unix time) lastoccurence

Closes #20804

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

apps/dav/lib/CalDAV/CalDavBackend.php

@@ -2814,7 +2814,7 @@ public function getDenormalizedData($calendarData) {
 			'size' => strlen($calendarData),
 			'componentType' => $componentType,
 			'firstOccurence' => is_null($firstOccurrence) ? null : max(0, $firstOccurrence),
-			'lastOccurence' => $lastOccurrence,
+			'lastOccurence' => is_null($lastOccurrence) ? null : max(0, $lastOccurrence),
 			'uid' => $uid,
 			'classification' => $classification
 		];