preproc: Copy rate-each/@dim only if it exists

If it is copied an yields an empty value, then it is later converted into a
0, which is wholly incorrect in the case of generators.

* src/current/include/preproc/macros.xsl (preproc:macros)[lv:rate-each]:
  Conditionally copy `@dim'.

src/current/include/preproc/macros.xsl

@@ -403,7 +403,11 @@
 
     <xsl:sequence select="./lv:class" />
 
-    <c:sum of="_CMATCH_" index="{@index}" sym="{@gensym}" dim="{@dim}">
+    <c:sum of="_CMATCH_" index="{@index}" sym="{@gensym}">
+      <xsl:if test="@dim">
+        <xsl:copy-of select="@dim" />
+      </xsl:if>
+
       <!-- copy @generates, if it exists (has the benefit of copying nothing
            if it does not exist) -->
       <xsl:sequence select="@generates" />