conditionally load a default static image if image metabox has no value

i have this basic while loop for my posts

<?php

         if ( have_posts() ) : while ( have_posts() ) : the_post();

         $title = get_the_title();

         $article = apply_filters( 'the_content', get_the_content() );

         $date = get_the_date();

         $source = get_post_meta( get_the_ID(), '_articles_url', true );

         $img = wp_get_attachment_image( get_post_meta( get_the_ID(), '_articles_image_id', 1 ), 'full', 1, get_post_meta($attachment_id, '_wp_attachment_image_alt', true) );


    ?>

i want to write a conditional to check if $img has a value or not , and to load a static default image in case of $img is empty.