How do I set a plain text field value to be empty?

<?php
function MY_MODULE_node_update(NodeInterface $node)
{
    if ($node instanceof \Drupal\node\NodeInterface) {

        $nid = $node->id();
        $nodeType = $node->getType();
    
        $vid = \Drupal::entityTypeManager()
                ->getStorage('node')
                ->getLatestRevisionId($nid);

        $comments = $node->get('field_workflow_comment')->getValue();

        foreach($comments as $comment){
            $revision_log = ($comment['value']);
        }

        // $empty = array();
        // $commentEmpty = $node->set('field_workflow_comment', $empty);
        
        $node->field_workflow_comment = [];

        $timestamp = REQUEST_TIME;

        $connection = Database::getConnection();

        $add_revision_log = $connection->update('node_revision')
                            ->fields([
                                'revision_log' => $revision_log
                            ])
                            ->condition('nid', $nid)
                            ->condition('vid', $vid)
                            ->condition('revision_timestamp', $timestamp)
                            ->execute(); 
        $node->save();
     
    }
}

This does not work for me on hook_node_update()

Thanks in advance