Crossref DOI deposit records can be created (and "successfully" submitted) with no DOIs in them

[rhigman]

While compiling Crossref DOI deposit output, if a work has no DOI, we only raise an error if it's a chapter, not the parent work.

https://github.com/thoth-pub/thoth/blob/6532713b89fd36ed734164c058d8aef175938da0/thoth-export-server/src/xml/doideposit_crossref.rs#L437-L444

This is because it's permissible to send DOI deposit files without a parent work DOI, e.g. if the only purpose of the submission is to register the chapter DOIs. But if a work has no DOI and also no chapters, we don't flag this, but instead create a DOI deposit file that doesn't contain any DOIs (hence does nothing).

Crossref will happily accept this file, and send a confirmation email which doesn't indicate any failures, just has an empty record_count (as below).

<?xml version="1.0" encoding="UTF-8"?>
<doi_batch_diagnostic status="completed" sp="ds5">
   <submission_id>[...]</submission_id>
   <batch_id>[...]</batch_id>
   <batch_data>
      <record_count>0</record_count>
      <success_count>0</success_count>
      <warning_count>0</warning_count>
      <failure_count>0</failure_count>
   </batch_data>
</doi_batch_diagnostic>

Relevant to #551.