Returning empty collection or a null?

Should a null value for an expected collection be preferred or an empty Collection object? Lets see through a program:

import java.util.Collection;
import java.util.Vector;

import static java.lang.System.out;

public class E {

  public void isNull (Collection col) {
       long t = System.nanoTime();
       if (col == null) {
       }
       out.println ("Time to check null: " + (System.nanoTime () - t));
  }

  public void isEmpty (Collection col) {
     long t = System.nanoTime ();
     if (col.isEmpty ()) {
     }
     out.println ("Time to check empty: " + (System.nanoTime () - t));
  }

  public static void main (String args []) {
     E e = new E ();
     e.isNull ((Collection) null);
     e.isEmpty (new Vector ());
  }

}

Running this on my laptop with Java 6 produced:

Time to check null:  45234
Time to check empty: 10750

Something to think about..