JMicrodata update

— Updated the types.json file with the new available Types and Properties
— Small documentation & code refactoring
— Bug fix: if specified, return an empty string on fallback or disabled library, even if a content is available https://github.com/joomla/joomla-cms/pull/4108/files#diff-cd3328236e7aecb09373c1c564b9bbb8R256
Actually, I was thinking the same :)