Do not create empty tags and remove tags only when necessary

app/Helper/TaskHelper.php

@@ -70,6 +70,7 @@ class TaskHelper extends Base
         $options = $this->tagModel->getAssignableList($project['id']);
 
         $html = $this->helper->form->label(t('Tags'), 'tags[]');
+        $html .= '<input type="hidden" name="tags[]" value="">';
         $html .= '<select name="tags[]" id="form-tags" class="tag-autocomplete" multiple>';
 
         foreach ($options as $tag) {

app/Model/TaskModificationModel.php

@@ -108,8 +108,6 @@ class TaskModificationModel extends Base
         if (isset($values['tags'])) {
             $this->taskTagModel->save($original_task['project_id'], $values['id'], $values['tags']);
             unset($values['tags']);
-        } else {
-            $this->taskTagModel->save($original_task['project_id'], $values['id'], array());
         }
     }
 }

app/Model/TaskTagModel.php

@@ -82,6 +82,7 @@ class TaskTagModel extends Base
     public function save($project_id, $task_id, array $tags)
     {
         $task_tags = $this->getList($task_id);
+        $tags = array_filter($tags);
 
         return $this->associateTags($project_id, $task_id, $task_tags, $tags) &&
             $this->dissociateTags($task_id, $task_tags, $tags);