this configuration is optional.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@5652 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/tasks/BuildTrigger.java

@@ -185,7 +185,7 @@ public class BuildTrigger extends Publisher {
         public Publisher newInstance(StaplerRequest req, JSONObject formData) throws FormException {
             return new BuildTrigger(
                 formData.getString("childProjects"),
-                formData.getBoolean("evenIfUnstable"));
+                formData.has("evenIfUnstable") && formData.getBoolean("evenIfUnstable"));
         }
 
         @Override