Wordpress if custom field value is null previous posts' value is displayed - custom-fields

Question

I have this code with a loop that gives me some trouble and strange results.
This particular post type has a few custom fields which I put in an array. Everything works as expected, but if a field has no value it will take the value from the previous post within the loop.
Let's say I have these posts in Wordpress:
Post ID 10
custom_1 = 10
custom_2 = 20
custom_3 = 30
Post ID 20
custom_1 = 40
custom_2 = null
custom_3 = null
If I run my loop I get these results
Post ID 10
custom_1 = 10
custom_2 = 20
custom_3 = 30
Post ID 20
custom_1 = 40
custom_2 = 20 (instead of null)
custom_3 = 30 (instead of null)
Here's the short version of that loop:
$query = new WP_Query($query_arg);
if ($query->have_posts()) {
while ($query->have_posts()) {
$query->the_post();
$result[] = array(
"custom_1" => get_post_meta($post->ID, 'custom_1', true),
"custom_2" => get_post_meta($post->ID, 'custom_2', true),
"custom_3" => get_post_meta($post->ID, 'custom_3', true)
);
}
}
wp_reset_postdata();
I can't seem to wrap my head around this one.. tried pretty much everything, but nothing seams to work.
Does anyone know what's happening here?
UPDATE:
Fixed it by changing the loop to this.
$query = new WP_Query($query_arg);
if ($query->have_posts()) {
while ($query->have_posts()) {
$query->the_post();
//set vars to "" which 'resets' the value with every new post in the loop
$custom_1 = "";
$custom_2 = "";
$custom_3 = "";
//set vars to the value of the custom fields
$custom_1 => get_post_meta($post->ID, 'custom_1', true);
$custom_2 => get_post_meta($post->ID, 'custom_2', true);
$custom_3 => get_post_meta($post->ID, 'custom_3', true);
$result[] = array(
"custom_1" => $custom_1,
"custom_2" => $custom_2,
"custom_3" => $custom_3
);
}
}
wp_reset_postdata();

Answer 1

It might be due to how the MYSQL handles null. Null means an absence of value, so they do not get a space in the database, not even an empty one. An empty string however does show in the database.
Try the following
Post ID 10
custom_1 = 10
custom_2 = 20
custom_3 = 30
Post ID 20
custom_1 = 40
custom_2 = ""
custom_3 = ""