#14355: Write failing collection test

Author: maxbucknell

dev/tests/integration/testsuite/Magento/Catalog/Model/ResourceModel/Product/CollectionTest.php

@@ -149,4 +149,20 @@ public function testJoinTable()
 
         self::assertContains($expected, str_replace(PHP_EOL, '', $sql));
     }
+
+    /**
+     * Checks that a collection uses the correct join when filtering by null.
+     *
+     * This actually affects AbstractCollection, but inheritance yada yada.
+     *
+     * @magentoDataFixture Magento/Catalog/Model/ResourceModel/_files/product_simple.php
+     * @magentoDbIsolation enabled
+     */
+    public function testFilterByNull()
+    {
+        $this->collection->addAttributeToFilter([['attribute' => 'special_price', 'null' => true]]);
+        $productCount = $this->collection->count();
+
+        $this->assertEquals(1, $productCount, 'Product with null special_price not found');
+    }
 }