Clarify PermissionGroup.owner is @Nonnull

Permission.owner is already marked @Nonnull, and
is built using PermissionGroup.owner.

core/src/main/java/hudson/security/PermissionGroup.java

@@ -30,6 +30,8 @@ import java.util.List;
 import java.util.SortedSet;
 import java.util.TreeSet;
 import javax.annotation.CheckForNull;
+import javax.annotation.Nonnull;
+
 import org.jvnet.localizer.Localizable;
 
 /**
@@ -53,7 +55,7 @@ public final class PermissionGroup implements Iterable<Permission>, Comparable<P
      * @param title sets {@link #title}
      * @throws IllegalStateException if this group was already registered
      */
-    public PermissionGroup(Class owner, Localizable title) throws IllegalStateException {
+    public PermissionGroup(@Nonnull Class owner, Localizable title) throws IllegalStateException {
         this.owner = owner;
         this.title = title;
         register(this);