Replace null as array key with an empty string

Using null as an array key is deprecated in PHP-8.5,
an empty string can be used for identical behavior.
This includes values that are passed to ZendForm::addDisplayGroup as elements,
as they will be used as array keys.

application/forms/BpConfigForm.php

@@ -66,7 +66,7 @@ class BpConfigForm extends BpConfigBaseForm
                     . ' this process should be retrieved from'
                 ),
                 'multiOptions' => array(
-                        null => $this->translate('Use the configured default backend'),
+                        '' => $this->translate('Use the configured default backend'),
                     ) + $this->listAvailableBackends()
             ));
         }

application/forms/SimulationForm.php

@@ -127,7 +127,7 @@ class SimulationForm extends BpConfigBaseForm
     protected function enumStateNames()
     {
         $states = array(
-            null => sprintf(
+            '' => sprintf(
                 $this->translate('Use current state (%s)'),
                 $this->translate($this->node->getStateName())
             )

library/Businessprocess/BpConfig.php

@@ -605,7 +605,7 @@ class BpConfig
     public function listInvolvedConfigs(&$configs = null)
     {
         if ($configs === null) {
-            $configs[$this->getName()] = $this;
+            $configs[$this->getName() ?? ''] = $this;
         }
 
         foreach ($this->importedNodes as $node) {

library/Businessprocess/Web/Form/QuickBaseForm.php

@@ -116,7 +116,7 @@ abstract class QuickBaseForm extends Zend_Form implements ValidHtml
             $nullLabel = $this->translate('- please choose -');
         }
 
-        return array(null => $nullLabel) + $enum;
+        return array('' => $nullLabel) + $enum;
     }
 
     protected function handleOptions($options = null)

library/Businessprocess/Web/Form/QuickForm.php

@@ -131,8 +131,8 @@ abstract class QuickForm extends QuickBaseForm
         );
 
         $grp = array(
-            $this->submitButtonName,
-            $this->deleteButtonName
+            $this->submitButtonName ?? '',
+            $this->deleteButtonName ?? ''
         );
         $this->addDisplayGroup($grp, 'buttons', array(
             'decorators' => array(