Show empty state when no to-do items

Requirement

• When the current user has no pending to-do items, show an empty state message that reads:

When an issue or merge request is assigned to you, or when you receive a @mention in a comment, this automatically triggers a new item in your To-Do List.

It's how you always know what to work on next.

• Ensure this text is localized
• See designs in &17929 (closed)