fix: handle missing AniList media format in episodes

[faddix]

Summary

Prevents nil pointer panics when creating episodes for AniList media
whose format field is null.
Tested with media https://anilist.co/anime/205068/One-Room-TA/.

AI assisted, with each passage thoroughly reviewed and edited as necessary.

Checklist

• I have read CONTRIBUTING.md
• I set up Seanime locally.
• I understand the code I changed.
• I tested the change locally.
• I wrote this PR description myself.
• I disclosed any AI assistance.
• I can explain and maintain this change.