code: public static Object evalNotNull(String tagName,
                                       String attributeName,
                                       String expression,
                                       Class expectedType,
                                       Tag tag,
                                       PageContext pageContext)
              throws JspException {
          if (expression != null) {
              Object r = ExpressionEvaluatorManager.evaluate(
                      attributeName, expression, expectedType, tag, pageContext);
              if (r == null) {
                  throw new NullAttributeException(tagName, attributeName);
              }
              return r;
          } else {
              return null;
          }
      }
docs: Evaluates an expression if present, but does not allow the expression
  to evaluate to 'null', throwing a NullAttributeException if it
  does.  The function <b>can</b> return null, however, if the
  expression itself is null.