Unable to recognise BigDecimal as Null in Custom tag

Question

I have written a tag for displaying monetary values which should display '-' if value is null but it always renders null values as 0.00
<%@ tag body-content="empty" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %> 
<%@ attribute name="value" required="true" type="java.math.BigDecimal" %>
<c:choose>
  <c:when test="${empty value}">-</c:when>
  <c:otherwise>  
    <fmt:formatNumber value="${value}" groupingUsed="true" minFractionDigits="${2}" maxFractionDigits="${2}"/>
  </c:otherwise> 
</c:choose>
<c:if test="${empty val[column.prop]}">NULL</c:if>                                                    
<mytag:fmtmoney value="${val[column.prop]}"/>First line just put in as a test
should display NULL- but displays NULL0.00

Answer 1

Ok I worked out what was happening but to be seems counter inuitive, the value being passed to the tag comes froma BigDecimal variable but if it is null it has no type. This null gets passed to the tag and the ExpressionLanguage Coercion rules convert it to zero, so in my tag cant differentiate between zero and null.
If I change the type of the attribute to a String it works because nulls get converted to empty String and ${empty var} returns true for empty Strings.
If I changed the type to a user defined class it works because user defined classes keep nulls as nulls.