xml: Fix pb_type attribute

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

convert-pb_type-attributes.xsl

diff --git a/convert-pb_type-attributes.xsl b/convert-pb_type-attributes.xsl
index 75f31c4..0f497ee 100644
--- a/convert-pb_type-attributes.xsl
+++ b/convert-pb_type-attributes.xsl
@@ -3,24 +3,32 @@
 
   <xsl:include href="identity.xsl" />
 
-  <!--
-    Convert
-      <pb_type><blif_model>XXX</blif_model></pb_type>
-    to
-      <pb_type blif_model="XXX"></pb_type>
-    -->
-  <xsl:template match="pb_type/blif_model">
-    <xsl:attribute name="blif_model"><xsl:value-of select="text()"/></xsl:attribute>
+  <xsl:template match="pb_type">
+    <xsl:copy>
+      <!--
+        Convert
+          <pb_type><blif_model>XXX</blif_model></pb_type>
+        to
+          <pb_type blif_model="XXX"></pb_type>
+        -->
+      <xsl:if test="blif_model">
+        <xsl:attribute name="blif_model"><xsl:value-of select="blif_model/text()"/></xsl:attribute>
+      </xsl:if>
+      <!--
+        Convert
+          <pb_type><pb_class>XXX</pb_class></pb_type>
+        to
+          <pb_type class="XXX"></pb_type>
+        -->
+      <xsl:if test="pb_class">
+        <xsl:attribute name="class"><xsl:value-of select="pb_class/text()"/></xsl:attribute>
+      </xsl:if>
+      <xsl:apply-templates select="@*"/>
+      <xsl:apply-templates select="node()"/>
+    </xsl:copy>
   </xsl:template>
 
-  <!--
-    Convert
-      <pb_type><pb_class>XXX</pb_class></pb_type>
-    to
-      <pb_type class="XXX"></pb_type>
-    -->
-  <xsl:template match="pb_type/pb_class">
-    <xsl:attribute name="class"><xsl:value-of select="text()"/></xsl:attribute>
-  </xsl:template>
+  <xsl:template match="pb_type/blif_model"/>
+  <xsl:template match="pb_type/pb_class"/>
 
 </xsl:stylesheet>