Using xsl:if to determine if a node is empty.

I am fairly new to XML/XSL.  In fact I am writing my first XSL for a
production environment.  I am trying to determine if a node is empty so that
I can print a line or not.  I have tried several different variations of
template matching and xsl:if statements and have not been able to get
anywhere.  

Here is the latest thing that I have tried which seems to match evertime
through so the text is coming out even on empty tags.

<xsl:template match="VENDOR_ITEM_NUM">	
	<xsl:variable name="empty_tag"></xsl:variable>
	<tr>
		<td></td>
		<td>
			<font size="2">
			<xsl:if test="$empty_tag != .">
			<xsl:value-of select="concat('Vendor Item number:
',.)"/>
			</xsl:if>
			</font>
		</td>
	</tr>
</xsl:template>

Any help would be greatly appreciated.



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