Add a guard against missing projects in todos

What does this MR do and why?

Adds a guard against missing projects in todos.

Fixes Opening `/dashboard/todos` on Omnibus install g... (#388051 - closed).

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

This is a bit tricky to validate, because todos with neither a group nor a project are invalid. It's unclear how the users that reported this got such todos in their database. Possibly a project/group got deleted in a way that their dependent todos didn't get cleaned up 🤷

To take the screenshot above, I did this:

• Added a = todo.inspect into app/views/dashboard/todos/_todo.html.haml to get the id of one of my pending todos.
• In the console, Todo.find(6).update_attribute(:project_id, nil). Same for group_id.
• Go to /dashboard/todos.