[7.5] AbstractNumbering.multilevel_type return => nil

[askonev]

docx.elements[0].numbering.abstruct_numbering.multilevel_type return nil

https://github.com/ONLYOFFICE/ooxml_parser/blob/d3c1d004d16d2e1c1e10e50ab83765de59aef600/lib/ooxml_parser/docx_parser/document_structure/numbering/abstract_numbering.rb#L36