further fix traceback on invalid .hgignore patterns
Please apply this little fix (matchfn return None if no patterns are
specified).
<item>
<title>#desc|strip|firstline|rstrip|escape#</title>
<link>#url#?cs=#node|short#</link>
<description><![CDATA[#desc|strip|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>