(PART) or (PART\*) does not work for grid style

[lief-y]

The problem mentioned in Parts and TOC links #22 still exists for the grid style. When using (PART*), not only there was an empty page created with the title (PART*) part title. The characters (PART*) also shows in the TOC. It seems a problem when build_chapter is called.

Can you please resolve this problem so that it only shows the part title as a unlinked list in TOC as a separator for chapters?

Thank you!