fix: properly parse dates or null

Resolves https://kolaente.dev/vikunja/frontend/issues/2214

src/helpers/parseDateOrNull.ts

@@ -1,13 +1,14 @@
 /**
  * Make date objects from timestamps
-*/
+ */
 export function parseDateOrNull(date) {
 	if (date instanceof Date) {
 		return date
 	}
 
-	if (date && !date.startsWith('0001')) {
+	if ((typeof date === 'string' || date instanceof String) && !date.startsWith('0001')) {
 		return new Date(date)
 	}
+
 	return null
 }