Remove empty rows from the database

I’d imagine those rows with empty content are more often then not revisions rather than trashed posts. To make sure you don’t delete stuff you want to keep I suggest you add the post_status to your SQL, using trash and auto-draft as value – the latter are the revisions. Btw the table column is called post_content not content. So your SQL would look something along the lines of the following:

DELETE FROM wp_posts
  WHERE post_content = ""
  AND   post_status = "trash"
  OR    post_status = "auto-draft";

You could of course use $wpdb and its delete method to achieve the same.