functions.php

class my_walker_nav_menu extends Walker_Nav_Menu {

    static $count=0;

    function start_el(&$output, $item, $depth, $args) { 

        global $wp_query;

        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
        $class_names = ' class="' . esc_attr( $class_names ) . '"';

        $attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target ) ? ' target="' . esc_attr( $item->target ) .'"' : '';
        $attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) .'"' : '';
        $attributes .= ! empty( $item->url ) ? ' href="' . esc_attr( $item->url ) .'"' : '';

        if ($depth==0) self::$count=0;  // reset var when we are in first level

        $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';
        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';


        if ($depth==1 && self::$count==1) {  // if we are in submenu and items count is 1...

        $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';
        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';    

        }

        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );

        self::$count++;  // increase counter
    }


}