wp/app/themes/twentyfifteen/sidebar.php

<?php
/**
 * The sidebar containing the main widget area.
 *
 * @package WordPress
 * @subpackage Twenty_Fifteen
 * @since Twenty Fifteen 1.0
 */

if ( has_nav_menu( 'primary' ) || has_nav_menu( 'social' ) || is_active_sidebar( 'sidebar-1' )  ) : ?>
	<div id="secondary" class="secondary">
		<?php if ( has_nav_menu( 'primary' ) ) : ?>
		<nav id="site-navigation" class="main-navigation" role="navigation">
			<?php
				wp_nav_menu( array(
					'menu_class'     => 'nav-menu',
					'theme_location' => 'primary',
				) );
			?>
		</nav><!-- .main-navigation -->
		<?php endif; ?>

		<?php if ( has_nav_menu( 'social' ) ) : ?>
		<nav id="social-navigation" class="social-navigation" role="navigation">
			<?php
				wp_nav_menu( array(
					'theme_location' => 'social',
					'depth'          => 1,
					'link_before'    => '<span class="screen-reader-text">',
					'link_after'     => '</span>',
				) );
			?>
		</nav><!-- .social-navigation -->
		<?php endif; ?>

		<?php if ( is_active_sidebar( 'sidebar-1' ) ) : ?>
		<div id="widget-area" class="widget-area" role="complementary">
			<?php dynamic_sidebar( 'sidebar-1' ); ?>
		</div><!-- .widget-area -->
		<?php endif; ?>
	</div><!-- .secondary -->
<?php endif; ?>