Fix field cardinality logic.

Closes #3485855

I'm new to this module, but this line stood out in the diff from the latest version and it seems like the logic was backwards.

• 1.2.24...1.2.25

I can also confirm that my node is able to be saved after this fix.

src/Plugin/Field/FieldWidget/TagifySelectWidget.php

@@ -150,15 +150,18 @@ class TagifySelectWidget extends OptionsWidgetBase {
   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
     $element = parent::formElement($items, $delta, $element, $form, $form_state);
     $target_type = $this->getFieldSetting('target_type');
+
     // User field definition doesn't have fieldStorage defined.
     $cardinality = $target_type !== 'user'
       ? $items->getFieldDefinition()->getFieldStorageDefinition()->isMultiple()
       : '';
     $tags_identifier = $items->getName();
+
     // Concat element position to the Tagify identifier.
     if (!empty($element['#field_parents'][1])) {
       $tags_identifier .= '_' . $element['#field_parents'][1];
     }
+
     $element += [
       '#type' => 'select_tagify',
       '#options' => $this->getOptions($items->getEntity()),
@@ -168,7 +171,7 @@ class TagifySelectWidget extends OptionsWidgetBase {
         'class' => [$tags_identifier],
       ],
       // Do not display a 'multiple' select box if there is only one option.
-      '#multiple' => ($this->multiple && count($this->options) > 1) || !$cardinality,
+      '#multiple' => ($this->multiple && count($this->options) > 1),
       '#cardinality' => $items->getFieldDefinition()
         ->getFieldStorageDefinition()
         ->getCardinality(),
@@ -178,6 +181,11 @@ class TagifySelectWidget extends OptionsWidgetBase {
       '#identifier' => $tags_identifier,
     ];
 
+    // Only add '#empty_value' if the field is not mandatory.
+    if (!$cardinality && !$items->getFieldDefinition()->isRequired()) {
+      $element['#empty_value'] = 0;
+    }
+
     // Add description if it doesn't exist.
     if ($target_type) {
       $entity_definition = $this->entityTypeManager->getDefinition($target_type);