Fix AssemblyPresenter

[takahashim]

🎩 What? Why?

NoMethodError in Decidim::Assemblies::Assemblies#showを出さないようにする修正です。

📌 Related Issues

• Related to wip: update decidim 0.28.0 codeforjapan/decidim-cfj#596

📋 Subtasks

• Add CHANGELOG upgrade notes, if required
• If there's a new public field, add it to GraphQL API
• Add documentation regarding the feature
• Add/modify seeds
• Add tests
• Another subtask

📷 Screenshots (optional)

[takahashim]

この件については decidim/decidim#12692 で報告済み＆ decidim/decidim#12693 でPR送付済みなんですが、リリースされるまでは独自対応が必要そうです /
This issue has already been reported at decidim/decidim#12692 and a pull request has been submitted at decidim/decidim#12693, but I don't think it will be included in the release version anytime soon, so we may need to solve it on our own code for a while.

[takahashim]

@ayuki-joto コミット追加したのでこれでいけるはずです！