Reverted casting JsonExpression(null) to SQL NULL

framework/db/mysql/JsonExpressionBuilder.php

@@ -34,9 +34,6 @@ class JsonExpressionBuilder implements ExpressionBuilderInterface
     public function build(ExpressionInterface $expression, array &$params = [])
     {
         $value = $expression->getValue();
-        if ($value === null) {
-            return 'NULL';
-        }
 
         if ($value instanceof Query) {
             list ($sql, $params) = $this->queryBuilder->build($value, $params);

tests/framework/db/mysql/QueryBuilderTest.php

@@ -205,7 +205,7 @@ class QueryBuilderTest extends \yiiunit\framework\db\QueryBuilderTest
             ],
             'null value' => [
                 ['=', 'jsoncol', new JsonExpression(null)],
-                '[[jsoncol]] = NULL', []
+                '[[jsoncol]] = CAST(:qp0 AS JSON)', [':qp0' => 'null']
             ],
             'null as array value' => [
                 ['=', 'jsoncol', new JsonExpression([null])],