BUG: Missing document node leads to exception when opening `/neos/content?node=...`

[grebaldi]

Description

When a user opens the UI directly via /neos/content?node=... and the document node (as identified by the node query parameter in the URL) cannot be found, the user is presented with an exception.

It is hard for the user to escape this. They would need to remove the node query parameter from the URL.

Steps to reproduce:

1. Create a new document node
2. Discard workspace via ./flow workspace:discard [workspace-name], while leaving the UI open
3. Reload the UI (via Browser refresh)
4. Exception happens

Expected Behavior

In Neos 8.3, the user is automatically redirected to the site node, if the document node cannot be found. Neos 9.0 should do the same thing.

Affected Versions

Neos: 9.0
UI: 9.0

[mhsdesign]

Strongly related to neos/neos-development-collection#4345