bugfix: time editor failed when no end time was set

core/src/main/java/de/srsoftware/umbrella/core/model/Time.java

@@ -115,6 +115,7 @@ public class Time implements Mappable{
 		}
 		if (json.has(START_TIME) && json.get(START_TIME) instanceof Number st) start = st.longValue();
 		if (json.has(END_TIME) && json.get(END_TIME) instanceof Number e) end = e.longValue();
+        if (end == 0) end = null;
 		if (end != null && end < start) throw UmbrellaException.invalidFieldException(END_TIME,"after start_time");
 		return this;
 	}