Query not returning posts with null custom fields

I have wordpress site with a few hundred posts, all categorized and tagged for years. I am splitting them up into “news” and “columns”, and have created a new true/false field, “is_column”; I want to run a query to display posts with is_column = 1 in one area of the front page, and posts with is_column as null to display in another.

one half — displaying columns — works fine:

// Create a meta query to filter posts by the custom field "is_column"

$meta_query = array(
    array(
      'key' => 'is_column',
      'value' => '1',
      'compare' => '=='
    )
  );
  // Create an array of arguments
  $args = array(
    'post_type' => 'post', // Post type
    'posts_per_page' => 4, // Number of posts to return
    'meta_query' => $meta_query, // The meta query created above
    'orderby' => 'post_date', // Order by post date
    'order' => 'DESC', // Order from newest to oldest
    'ignore_sticky_posts' => false // Include sticky posts at the top
  );
  // Create a new WP_Query object with the arguments
  $query = new WP_Query($args);
  // Check if the query has any posts
  if ($query->have_posts()) {
    // Loop through the posts
    while ($query->have_posts()) {
      // Set up the post data
      $query->the_post();
      // Display the post title
      echo '<a href="'. get_the_permalink() .'">';
      the_title(); // Change this from the_content() to the_title()
      echo '</a>';
    }
    // Restore the original post data
    wp_reset_postdata();
  } else {
    // No posts found
    echo 'No posts found with the custom field "is_column" set to true.';
  }

However, when I invert the query like this:

$meta_query = array(
    array(
      'key' => 'is_column',
      'value' => '1',
      'compare' => '!='
    )
  );

or even like this:

$meta_query = array(
    array(
      'key' => 'is_column',
      'value' => NULL,
      'compare' => '=='
    )
  );

…it falls back to ‘No posts found’.

How can I run this query?

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