Wrong element in DOI deposits for books in series without ISSN

[ja573]

When submitting a book to Crossref, the deposit fails with a schema validation error if the book belongs to a series that does not have an ISSN. The error from Crossref is:

Error validating schema crossref5.3.1.xsd : Error: cvc-complex-type.2.4.a: Invalid content was found starting with element 'contributors'. One of '{"http://www.crossref.org/schema/5.3.1":series_metadata}' is expected.

The root cause appears to be that book_series_metadata is applied to the parent record before validating that the series has at least one ISSN. Crossref requires that series_metadata includes at least one ISSN, and currently Thoth allows registering book series without an ISSN.

Relevant code sections:

• Validation before deposit: doideposit_crossref.rs#L788
• Metadata applied too early: doideposit_crossref.rs#L103