bugfix: Task.tags failed when null was passed

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

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

@@ -220,7 +220,7 @@ public class Task implements Mappable {
 
 	public Task tags(Collection<String> newValue){
 		tags.clear();
-		tags.addAll(newValue);
+		if (newValue != null) tags.addAll(newValue);
 		return this;
 	}