fix(MangaUpdates): handle missing thumbnail url

[IntermittentlyRupert]

Summary

Some MangaUpdates manga don't have images. Currently, the extension handles this by falling back to an empty string for the thumbnailUrl, which causes an error.

This PR switches to using an actually valid fallback thumbnailUrl.

Checklist

Making changes

• Follows Inkdex's Contributing Guidelines.

For updates to existing extensions

• Bumped the version value in pbconfig.ts for each modified extension.

For new extensions

• Generated tests with npx paperback-cli test --generate EXTENSION_NAME.

Testing changes

• npm run conformance passes.
• npm test -- EXTENSION_NAME passes.
• Bundled the extension and verified it works in the Paperback app.

Committing changes

• Commit messages follow the existing convention (type(Scope): summary, e.g. fix(EXTENSION_NAME): ...).

AI assistance

Pick one:

• This PR contains no AI-assisted changes.
• This PR is AI-assisted. I manually reviewed every change and added an Assisted-by: AGENT_NAME:MODEL_VERSION trailer to each AI-assisted commit.

[niclimcy]

thanks!