|_Header| proxy on prior sectPr element or None if this is first section.

def _prior_headerfooter(self):
        """|_Header| proxy on prior sectPr element or None if this is first section."""
        preceding_sectPr = self._sectPr.preceding_sectPr
        return (
            None if preceding_sectPr is None
            else _Header(preceding_sectPr, self._document_part, self._hdrftr_index)
        )