Support some but not all social links in theme mod

Change social media links section to this code and people can leave URIs blank in the customizer for services they choose not to use.

<?php if( get_theme_mod( 'iconic_one_social_activate' ) == '1') : ?>
        <div class="socialmedia">
            <?php if ( get_theme_mod( 'twitter_url' ) ) : ?>
                <a href="<?php echo get_theme_mod( 'twitter_url', 'default_value' ); ?>" target="_blank">
                    <img src="<?php echo get_template_directory_uri(); ?>/img/twitter.png" alt="Follow us on Twitter"/>
                </a>
            <?php endif; ?>
            <?php if ( get_theme_mod( 'facebook_url' ) ) : ?>
                <a href="<?php echo get_theme_mod( 'facebook_url', 'default_value' ); ?>" target="_blank">
                    <img src="<?php echo get_template_directory_uri(); ?>/img/facebook.png" alt="Follow us on Facebook"/>
                </a>
            <?php endif; ?>
            <?php if ( get_theme_mod( 'plus_url' ) ) : ?>
                <a href="<?php echo get_theme_mod( 'plus_url', 'default_value' ); ?>" rel="author" target="_blank">
                    <img src="<?php echo get_template_directory_uri(); ?>/img/gplus.png" alt="Follow us on Google Plus"/>
                </a>
            <?php endif; ?>
            <?php if ( get_theme_mod( 'rss_url' ) ) : ?>
                <a href="<?php echo get_theme_mod( 'rss_url', 'default_value' ); ?>" target="_blank">
                    <img src="<?php echo get_template_directory_uri(); ?>/img/rss.png" alt="Follow us on rss"/>
                </a>
            <?php endif; ?>
        </div>
    <?php endif; ?>