Section CSS Class on Non-Doctypes

The Section Macro renders the doctype ID as part of the CSS class on the enclosing HTML DIV element. If the section is on a non-projectdoc page, this value resolves to 'null' so that the class 'projectdoc-null' is rendered.

The 'null' sequence may seem as a bug. Therefore we need to change the sequence to 'projectdoc-not-a-doctype'.

This change breaks the runtime if users have specified CSS styles where the use the old 'projectdoc-null' CSS class.