You can use something like this (but you didn't say if you are using
XSLT 1.0 or 2.0):

   <xsl:template match="C">
     <xsl:variable name="empty-t" as="element()">
       <T>*</T>
     </xsl:variable>
     <table-line>
       <xsl:apply-templates select="
           P, if ( T ) then T else $empty-t, S"/>
     </table-line>
   </xsl:template>

   <xsl:template match="P|T|S">
     <table-cell>
       <xsl:value-of select="."/>
     </table-cell>
   </xsl:template>