checkbox: value for unchecked will be saved now.

extension/Classes/Core/AbstractBuildForm.php

@@ -2059,7 +2059,6 @@ abstract class AbstractBuildForm {
 
         if ($formElement[FE_CHECKBOX_CHECKED] === $value) {
             $attribute .= Support::doAttribute('checked', 'checked');
-//            $valueJson = true;
             $valueJson = $value;
         }
 

extension/Classes/Core/Store/FillStoreForm.php

@@ -257,7 +257,7 @@ class FillStoreForm {
             switch ($formElement[FE_TYPE]) {
                 case FE_TYPE_CHECKBOX:
                     // Checkbox Multi: collect values
-                    $val = $this->collectCheckBoxValues($clientFieldName, $clientValues);
+                    $val = $this->collectCheckBoxValues($clientFieldName, $clientValues, $formElement[FE_CHECKBOX_CHECKED] ?? '');
                     if ($val !== false) {
                         $clientValues[$clientFieldName] = $val;
                     }
@@ -352,7 +352,7 @@ class FillStoreForm {
      *
      * @return string
      */
-    private function collectCheckBoxValues($clientFieldName, array $clientValues) {
+    private function collectCheckBoxValues($clientFieldName, array $clientValues, $unchecked) {
 
         // Check for Single
 //        $checkboxKey = HelperFormElement::prependFormElementNameCheckBoxMulti($clientFieldName, '', false);
@@ -366,7 +366,7 @@ class FillStoreForm {
             return $clientValues[$clientFieldName];
         }
 
-        return false;
+        return $unchecked;