Issue #1807058 by sun: Fixed Config stores a NULL value as an array.

core/lib/Drupal/Core/Config/Config.php

@@ -270,13 +270,14 @@ class Config {
    * Any non-scalar value that is not an array (aka objects) gets cast
    * to an array.
    *
-   * @param $value
+   * @param mixed $value
    *   A value being saved into the configuration system.
-   * @param $value
+   *
+   * @return string
    *   The value cast to a string or array.
    */
   public function castValue($value) {
-    if (is_scalar($value)) {
+    if (is_scalar($value) || $value === NULL) {
       // Handle special case of FALSE, which should be '0' instead of ''.
       if ($value === FALSE) {
         $value = '0';

core/modules/config/lib/Drupal/config/Tests/ConfigFileContentTest.php

@@ -84,6 +84,9 @@ class ConfigFileContentTest extends WebTestBase {
     // Add a boolean true value. Should get cast to 1
     $config->set($true_key, TRUE);
 
+    // Add a null value. Should get cast to an empty string.
+    $config->set('null', NULL);
+
     // Add an array with a nested boolean false that should get cast to 0.
     $config->set($casting_array_key, $casting_array_value);
     $config->save();
@@ -119,6 +122,9 @@ class ConfigFileContentTest extends WebTestBase {
     // Read true value
     $this->assertEqual($config->get($true_key), '1', format_string("Boolean TRUE value returned the string '1'."));
 
+    // Read null value.
+    $this->assertIdentical($config->get('null'), '');
+
     // Read false that had been nested in an array value
     $this->assertEqual($config->get($casting_array_false_value_key), '0', format_string("Nested boolean FALSE value returned the string '0'."));