code: public static boolean isNull(Object s)
      {
          if (s == null)
          {
              return true;
          }
  
          if (!(s instanceof String))
          {
              return false;
          }
  
          return NULL_LITERALS.contains(((String) s).toLowerCase(Locale.US));
      }
docs: Checks if the value of s was <code>null</code> in Asterisk. <br>
  This method is useful as Asterisk likes to replace <code>null</code>
  values with different string values like "unknown", "&lt;unknown&gt;" or
  "&lt;null&gt;". <br>
  To find such replacements search for <code>S_OR</code> in Asterisk's
  source code. You will find things like
  
  <pre>
  S_OR(chan-&gt;cid.cid_num, "&lt;unknown&gt;")
  fdprintf(fd, "agi_callerid: %s\n", S_OR(chan-&gt;cid.cid_num, "unknown"));
  </pre>
  
  and more...
  
  @param s the string to test, may be <code>null</code>. If s is not a
  string the only test that is performed is a check for
  <code>null</code>.
  @return <code>true</code> if the s was <code>null</code> in Asterisk;
  <code>false</code> otherwise.