Don't add the inclusion error when tracker is not set, the blank error is enough.

git-svn-id: http://svn.redmine.org/redmine/trunk@15492 e93f8b46-1217-0410-a6f0-8f06a7374b81

Author: jplang

app/models/issue.rb

@@ -691,7 +691,7 @@ def validate_issue
 
     # Checks that the issue can not be added/moved to a disabled tracker
     if project && (tracker_id_changed? || project_id_changed?)
-      unless project.trackers.include?(tracker)
+      if tracker && !project.trackers.include?(tracker)
         errors.add :tracker_id, :inclusion
       end
     end