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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