R1:b32b84b64515

Fix exception for anonymous viewers on dashboard query panels with user-specific data

Summary:
When a viewer is not logged in and opens a public dashboard which embeds a query panel whose data requires a Current Viewer context (e.g. assigned or authored tasks, or joined projects), do not throw an exception but show an explanatory placeholder sentence (similar to already existing 'No tasks found.' or 'No documents found.' strings).

```
EXCEPTION: (Exception) Query "assigned" is unknown to application…