introduce hudson.Util#fixNull(value, defaultValue)

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

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

@@ -1195,8 +1195,14 @@ public class Util {
      */
     @Nonnull
     public static String fixNull(@CheckForNull String s) {
-        if(s==null)     return "";
-        else            return s;
+        return fixNull(s, "");
+    }
+
+    /**
+     * Convert null to defaultValue.
+     */
+    public static <T> T fixNull(@CheckForNull T s, T defaultValue) {
+        return s != null ? s : defaultValue;
     }
 
     /**
@@ -1221,22 +1227,22 @@ public class Util {
 
     @Nonnull
     public static <T> List<T> fixNull(@CheckForNull List<T> l) {
-        return l!=null ? l : Collections.<T>emptyList();
+        return fixNull(l, Collections.<T>emptyList());
     }
 
     @Nonnull
     public static <T> Set<T> fixNull(@CheckForNull Set<T> l) {
-        return l!=null ? l : Collections.<T>emptySet();
+        return fixNull(l, Collections.<T>emptySet());
     }
 
     @Nonnull
     public static <T> Collection<T> fixNull(@CheckForNull Collection<T> l) {
-        return l!=null ? l : Collections.<T>emptySet();
+        return fixNull(l, Collections.<T>emptySet());
     }
 
     @Nonnull
     public static <T> Iterable<T> fixNull(@CheckForNull Iterable<T> l) {
-        return l!=null ? l : Collections.<T>emptySet();
+        return fixNull(l, Collections.<T>emptySet());
     }
 
     /**