Fix solr-updater and book page errors when invalid identifier

[cdrini]

Part of #12546 . Handles making solr-updater and the book page resilient to None identifiers.

Technical

Testing

Now renders correctly on testing: https://testing.openlibrary.org/works/OL45259313W/Yely%27s_Holiday_Traditions

Screenshot

Stakeholders

[Copilot]

Pull request overview

This PR hardens Solr edition document building (and upstream identifier rendering) against None identifier values, preventing solr-updater crashes like those described in #12546.

Changes:

• Filter out None identifier values when building edition Solr identifier fields.
• Skip None identifier values when processing identifiers in the upstream models layer (prevents page/render-time errors).
• Add a focused unit test validating identifier key normalization and None filtering behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
openlibrary/solr/updater/edition.py	Filters None values out of edition identifier lists before calling .strip() and emitting Solr fields.
openlibrary/plugins/upstream/models.py	Skips None identifier values during _process_identifiers() to avoid downstream string operations failing.
openlibrary/tests/solr/updater/test_edition.py	Adds coverage for EditionSolrBuilder.identifiers with weird keys and None values.