OZG-4375 handle null values when converting date string

alfa-xdomea/src/main/java/de/ozgcloud/alfa/common/DateConverter.java

@@ -26,11 +26,13 @@ public class DateConverter {
 
 	public Optional<XMLGregorianCalendar> convertGermanFormatToISO(String dateStr) {
 		try {
+			if (nonNull(dateStr)) {
 				return Optional.of(createDatatypeFactory().newXMLGregorianCalendar(LocalDate.parse(dateStr, DATE_TIME_FORMATTER).toString()));
+			}
 		} catch (Exception e) {
 			LOG.warn("Date '{}' cannot be converted to ISO format.", dateStr, e);
-			return Optional.empty();
 		}
+		return Optional.empty();
 	}
 
 	public XMLGregorianCalendar toXmlGregorianCalendar(ZonedDateTime date) {