fix for null pointer exception that occurred when missing user-agent in request header

see also http://forum.yacy-websuche.de/viewtopic.php?f=6&t=78&hilit=

git-svn-id: https://svn.berlios.de/svnroot/repos/yacy/trunk@3943 6c8d7289-2bf4-0310-a012-ef5d649a1542

source/de/anomic/http/httpHeader.java

@@ -492,7 +492,7 @@ public final class httpHeader extends TreeMap implements Map {
     }
     
     public String userAgent() {
-        return (String) get(USER_AGENT);
+        return (String) get(USER_AGENT, "");
     }
     
     public long age() {