fix(series): add empty media items filter

[AntonLantukh]

Description

• Unpublished episodes are returned as null, we need to filter these items.

Steps completed:

According to our definition of done, I have completed the following steps:

• [ ] Acceptance criteria met
• [ ] Unit tests added
• [ ] Docs updated (including config and env variables)
• [ ] Translations added
• [ ] UX tested
• [ ] Browsers / platforms tested
• [ ] Rebased & ready to merge without conflicts
• [ ] Reviewed own code

[github-actions[bot]]

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