`header` content not rendered into the TOC if requirement is not `normative`

[opt12]

tl;dr;
The content of the header attribute of non-normative items is not rendered to the TOC entries.
The rendering of the header attribute to the TOC should be independent from the normative attribute.

Details
I created a requirements template with a lot of structuring elements which are basically headings for sub-chapters in the requirements document. They have the following properties:

• header contains the heading text
• level ends with a ".0" to indicate that it's a structuring element
• normative == false
• text property is mostly empty, as only the structuring headings are contained in the item.
  However, very few of these items have content in the text property with further explanations.

For testing, I also added two normative requirements.

When publishing to HTML (or to markdown or to whatever), the content is rendered quite nicely:

In the non-normative items, the requirements ID after the header is ommitted, which is understandable, as one doesn't need to references non-normative items.
The content of the header attribute is rendered as heading to the items.

In the TOC however, the non-normative items are not added with their header content, but it is left empty or the first line of the text attribute is added, if available.

From my point of view, the header field shall be added to the TOC entries independently from the state of the normative attribute.