have RSSFeed.getChannel return empty message on missing channel element,

a) required b) prevent NPE in rss servlets
+ add test

source/net/yacy/cora/document/feed/RSSFeed.java

@@ -36,7 +36,7 @@ public class RSSFeed implements Iterable<RSSMessage> {
     public static final int DEFAULT_MAXSIZE = 10000;
 
     // class variables
-    private RSSMessage channel = null;
+    private RSSMessage channel = null; // single required element  see http://www.rssboard.org/rss-profile#element-channel
     private final Map<String, RSSMessage> messages; // a guid:Item map
     private final int maxsize;
 
@@ -91,7 +91,15 @@ public class RSSFeed implements Iterable<RSSMessage> {
         this.channel = channelItem;
     }
 
+    /**
+     * Return Channel element
+     * (This element is required and must contain three child elements: description, link and title)
+     * see http://www.rssboard.org/rss-profile#element-channel
+     * @return RSSMessage with channel elements or empty RSSMessage
+     */
     public RSSMessage getChannel() {
+        // This element is required and must contain three child elements: description, link and title.
+        if (this.channel==null) this.channel = new RSSMessage();
         return this.channel;
     }
 

source/net/yacy/cora/document/feed/RSSMessage.java

@@ -325,6 +325,10 @@ public class RSSMessage implements Hit, Comparable<RSSMessage>, Comparator<RSSMe
         setValue(Token.description, description);
     }
 
+    /**
+     * set a URL that points to the documentation for the format used in the RSS file.
+     * @param docs e.g. "http://www.rssboard.org/rss-specification"
+     */
     @Override
     public void setDocs(final String docs) {
         setValue(Token.docs, docs);

test/java/net/yacy/cora/document/feed/RSSFeedTest.java

@@ -0,0 +1,22 @@
+package net.yacy.cora.document.feed;
+
+import java.io.IOException;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class RSSFeedTest {
+
+    /**
+     * Test of getChannel method, of class RSSFeed.
+     */
+    @Test
+    public void testGetChannel() throws IOException {
+        RSSFeed feed = new RSSFeed(Integer.MAX_VALUE);
+
+        // channel is required in RSS 2.0 and accessed in code w/o != null checks
+        RSSMessage channel = feed.getChannel();
+        assertNotNull(channel);
+
+    }
+
+}