Ensure node isn't new before attempting to load groups

This code:

 public function build() {
    // Get the associated group content for the current node.
    $node = $this->getContextValue('node');
    if ($node) {
      $group_contents = GroupContent::loadByEntity($node);

fails when on a node/add/whatever page because it gets an unsaved node object. This results in the error:

Drupal\Core\Entity\EntityStorageException: Cannot load GroupContent entities for an unsaved entity. in Drupal\group\Entity\Storage\GroupContentStorage->loadByEntity()