Allow to leave preset filter values empty

We fixed the bug that one could not filter for empty string, yet we
removed the possibility to not define a preset at all by doing so.
This restores the possibility by allowing "null" as value.

src/CoreBundle/Contao/Hooks/AbstractContentElementAndModuleCallback.php

@@ -387,10 +387,12 @@ protected function buildFilterParamsFor(DC_Table $dataContainer, $elementName)
             return;
         }
 
+        $translatedNull = $this->translator->trans('filter_option.null', [], self::$tableName);
+
         $collection = $this->filterFactory->createCollection($filterId);
         $dca        = $collection->getParameterDCA();
         foreach ($dca as $fieldName => $subField) {
-            $options = [];
+            $options = ['--null--' => $translatedNull];
             foreach (($subField['options'] ?? []) as $key => $value) {
                 $newKey = $this->loadCallback($key);
                 if (null !== $newKey) {
@@ -416,7 +418,7 @@ protected function buildFilterParamsFor(DC_Table $dataContainer, $elementName)
      */
     public function saveCallback(string $value = null)
     {
-        return null === $value ? null : base64_decode($value);
+        return null === $value ? '--null--' : base64_decode($value);
     }
 
     /**
@@ -428,7 +430,7 @@ public function saveCallback(string $value = null)
      */
     public function loadCallback(string $value = null)
     {
-        return null === $value ? null : trim(base64_encode($value), '=');
+        return null === $value ? '--null--' : trim(base64_encode($value), '=');
     }
 
     /**

src/CoreBundle/Resources/translations/tl_content.en.xlf

@@ -20,6 +20,9 @@
       <trans-unit id="editfiltersetting.description" resname="editfiltersetting.description">
         <source>Edit the filter setting ID %id%.</source>
       </trans-unit>
+      <trans-unit id="filter_option.null" resname="filter_option.null">
+        <source>- no value [null] -</source>
+      </trans-unit>
     </body>
   </file>
 </xliff>

src/CoreBundle/Resources/translations/tl_modules.en.xlf

@@ -20,6 +20,9 @@
       <trans-unit id="editfiltersetting.description" resname="editfiltersetting.description">
         <source>Edit the filter setting ID %id%.</source>
       </trans-unit>
+      <trans-unit id="filter_option.null" resname="filter_option.null">
+        <source>- no value [null] -</source>
+      </trans-unit>
     </body>
   </file>
 </xliff>