I'd agree with comments, best to maybe un-publish empty articles.
Also there is one possible flaw in this plan, and that is the assumption that the component being called is com_content with a single article. What if the page the menu item points to is not com_content, or if it is a blog view of com_content? In those instances you wont have a single article id.
These thoughts aside, you could do this with the following code...
# get the app object, you may already be doing this in your code
$app = JFactory::getApplication();
# get the article id
$article_id = $app->input->get('id');
# check we have one
if ($article_id) {
# load the JTableContent object
$article = JTable::getInstance('content');
$article->load($article_id);
# use these for debug
echo $article->get('title');
echo $article->get('introtext');
if ( strlen($article->get('introtext') ) ) {
echo "we have content :)";
?>
<article id="content">
<section id="mid">
<jdoc:include type="component" />
</section>
</article>
<?php
} else {
echo "we have no content :( ";
}
}
This uses the Joomla's JTableContent, see https://docs.joomla.org/Creating_content_using_JTableContent