Do not fail if known hosts file does not contain valid host key

KnownHosts (implementing HostKeyRepository) in Jsch can return null
which could cause NullPointerException in Stream.of(...)

Change-Id: Iddcf5f34f8c8475a85ca7ae018bbe48d1b3fbbc0
Signed-off-by: Lajos Olah <lajos.olah.jr@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/JschConfigSessionFactory.java

@@ -219,7 +219,13 @@ public abstract class JschConfigSessionFactory extends SshSessionFactory {
 
 	private static void setPreferredKeyTypesOrder(Session session) {
 		HostKeyRepository hkr = session.getHostKeyRepository();
-		List<String> known = Stream.of(hkr.getHostKey(hostName(session), null))
+		HostKey[] hostKeys = hkr.getHostKey(hostName(session), null);
+
+		if (hostKeys == null) {
+			return;
+		}
+
+		List<String> known = Stream.of(hostKeys)
 				.map(HostKey::getType)
 				.collect(toList());