news: Allow empty <section> elements

Creating dummy <change> elements was a workaround for the
HTML DTD not allowing empty <ul> elements, but we can do
better by tweaking the the XSLT stylesheet.

docs/news-html.xsl

@@ -57,9 +57,11 @@
       <strong>
         <xsl:value-of select="@title"/>
       </strong>
-      <ul>
-        <xsl:apply-templates select="change"/>
-      </ul>
+      <xsl:if test="*">
+        <ul>
+          <xsl:apply-templates select="change"/>
+        </ul>
+      </xsl:if>
     </li>
   </xsl:template>
 

docs/news.xml

@@ -23,19 +23,10 @@
 
   <release version="FIXME" date="unreleased">
     <section title="New features">
-      <change>
-        <summary/>
-      </change>
     </section>
     <section title="Improvements">
-      <change>
-        <summary/>
-      </change>
     </section>
     <section title="Bug fixes">
-      <change>
-        <summary/>
-      </change>
     </section>
   </release>
 
@@ -44,19 +35,10 @@
 <libvirt>
   <release version="v3.3.0" date="unreleased">
     <section title="New features">
-      <change>
-        <summary/>
-      </change>
     </section>
     <section title="Improvements">
-      <change>
-        <summary/>
-      </change>
     </section>
     <section title="Bug fixes">
-      <change>
-        <summary/>
-      </change>
     </section>
   </release>
   <release version="v3.2.0" date="2017-04-02">

docs/schemas/news.rng

@@ -35,21 +35,18 @@
       <attribute name="title">
         <data type="string"/>
       </attribute>
-      <oneOrMore>
+      <zeroOrMore>
         <ref name="change"/>
-      </oneOrMore>
+      </zeroOrMore>
     </element>
   </define>
 
   <define name="change">
     <element name="change">
       <element name="summary">
-        <choice>
-          <data type="string">
-            <param name="pattern">\n[^\n]+\n +</param>
-          </data>
-          <empty/>
-        </choice>
+        <data type="string">
+          <param name="pattern">\n[^\n]+\n +</param>
+        </data>
       </element>
       <optional>
         <element name="description">