Document without word/_rels/document.xml.rels raises NoMethodError on #paragraphs #192

Document#load_rels rescues a missing word/_rels/document*.xml.rels and leaves @rels nil. Afterwards, #hyperlink_relationships calls @rels.xpath without a nil check. That breaks #hyperlinks, #document_properties, and therefore #paragraphs / #tables / #to_s.

Repro (basic.docx with the rels part removed)

zip = Zip::File.open('spec/fixtures/basic.docx')
buf = Zip::OutputStream.write_buffer do |out|
  zip.each do |e|
    next if e.name.include?('_rels/document')
    out.put_next_entry(e.name)
    out.write(zip.read(e.name))
  end
end
doc = Docx::Document.open(buf.string)
doc.paragraphs
# => NoMethodError: undefined method 'xpath' for nil

Expected: a document without a relationships part opens and reads normally, and #hyperlinks returns {}. This mirrors the missing-styles.xml case fixed in #158.

Found while splitting the specs into per-class unit specs (#188).