/// <summary>
 /// Tests if a string is null or empty
 /// </summary>
 /// <param name="nullOr"></param>
 /// <param name="customMessage">Custom message to add to the final failure message</param>
 public static void Empty(
     this INullOr <string> nullOr,
     string customMessage
     )
 {
     nullOr.Empty(() => customMessage);
 }
 /// <summary>
 /// Tests if a string is null or empty
 /// </summary>
 /// <param name="nullOr"></param>
 public static void Empty(
     this INullOr <string> nullOr
     )
 {
     nullOr.Empty(NULL_STRING);
 }