Still allow header and footer embeds to be optional

New bug was introduction with the latest release. If you added content in the header embed, you were not able to save the page without adding content in the footer embed (vice-versa). These changes allow users to save their changes, even if they only have content in one of the embeds text box.

Also, additional validation has been added to make sure that each non self closing tag has a closing tag in the embeds and that the content in the embeds start with <