Reporting chokes if a chapter is empty

[LeamHall]

  File "/usr/local/bin/bookbot.py", line 440, in <module>
    collated_reports = collate_reports(book)
                       ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/bin/bookbot.py", line 376, in collate_reports
    chapter_reports.append(chapter.report_data)
                           ^^^^^^^^^^^^^^^^^^^
AttributeError: 'Chapter' object has no attribute 'report_data'