Fixes a bug in GoReply when no valid moves were found

src/main/java/com/fathzer/jchess/uci/GoReply.java

@@ -154,7 +154,7 @@ public String toString() {
 	 * @return The line or an empty optional if no information is available
 	 */
 	public Optional<String> getMainInfoString() {
-		return getInfoString(0);
+		return bestMove==null ? Optional.empty() : getInfoString(0);
 	}
 
 	/** Gets the uci info line to return just before sending the reply.