Better support for blocks with content that don't start with / contain `<div>` elements

[robaimes]

Blocks that don't contain a <div> element are being modified incorrectly. CMS block content does not always contain a <div> element.

• The block doesn't use PageBuilder, so <div> is not mandatory
• The block content has been added via setup script in which it does not contain a <div> element
  • If this is saved via the admin and PageBuilder is enabled, then this will fix it but requires manual intervention

To reproduce:

Create a CMS block via a setup script:

$cmsBlock = $this->blockInterfaceFactory->create();
$cmsBlock->setIdentifier('my_cool_new_cms_block');
    ->setTitle('My Cool New CMS Block')
    ->setContent('<p>My really cool new CMS block</p>')
    ->setData('stores', [0]);

$this->blockRepository->save($cmsBlock);

The content when rendered on the frontend is display as such:

M data-block-indentifier="my_cool_new_cms_block"y really cool new CMS block