When running dagit with --empty-workspace, the app just shows spinners on the left nav and main content. Repair this:
- Make the repo context nullable, and begin using loading state to distinguish between "loading" and "empty".
- Show "No repositories found" in left nav if there are no repos.
- Render "Runs" list, but with a limited context menu to prevent re-running pipelines or opening them in the playground.
- Ensure that the logs for a single run can be displayed.
In followups, we might consider modifying the top nav to be clearer about where the tabs will go when viewing a single run. Right now, the "Runs" tab is selected for both the Runs list and the individual Run view, and tab navigation could be a little confusing when viewing the single run.