adding the notion that a plugin can be pinned to avoid overwritten by a bundled plugin.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@21977 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/Util.java

@@ -80,6 +80,8 @@ import java.util.ResourceBundle;
 import java.util.SimpleTimeZone;
 import java.util.StringTokenizer;
 import java.util.Arrays;
+import java.util.Collections;
+import java.util.Set;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import java.util.regex.Matcher;
@@ -852,6 +854,14 @@ public class Util {
         return fixEmpty(s.trim());
     }
 
+    public static <T> List<T> fixNull(List<T> l) {
+        return l!=null ? l : Collections.<T>emptyList();
+    }
+
+    public static <T> Set<T> fixNull(Set<T> l) {
+        return l!=null ? l : Collections.<T>emptySet();
+    }
+
     /**
      * Cuts all the leading path portion and get just the file name.
      */