XSL Problem

In xsl, I have a sql query in a xml template. I view the results with a xsl file in html.
I like to use the next lines to test if the sql field named 'Year' was NULL I like to view a empty cell on my table. If not I like to see the value of the field:

<xsl:choose> 
  <xsl:when test="@Year = NULL">Empty</xsl:when>
  <xsl:otherwise><xsl:value-of select = '@Year'/></xsl:otherwise>
</xsl:choose>

What is the correct syntax to test 'NULL' value?

It's possible to force the border of my cell if the body is Empty?
Thank's a lot

andrea