Ekir 384 handle null contributor names

[natlibfi-kaisa]

Description

If an OPDS2 feed publication has an author but the name value in null, import the author's name as [Unknown] to Circulation.

Motivation and Context

When the name of a contributor was missing, it raised an error which prevented a book's import into Circulation. This PR changes the functionality so that

• When a name property of a webpub_manifest_parser Contributor object is missing, the missing properties are saved as [Unknown] values into Circulation's contributor data.
• Since the missing properties caused an error during the parser's extraction, that error is no longer recorded as a coverage fail which lead to the book not being imported to Circulation.
• The functionality for edited books without an author is kept intact.

How Has This Been Tested?

Based on unit tests since our test feed doesn't contain an appropriate item.

Checklist

• I have updated the documentation accordingly --> Backend documentation updated in Kupoli.
• All new and existing tests passed --> Added some test cases also for existing ones to clarify edited books' author handling.
• Transifex translators have been notified. ---> Not yet, only once we figure out how new or modified strings are uploaded to Transifex.

[NatLibFi-JoonaKupe]

A-ok