Make sure that all session contexts has unique IDs even if they don't have a kernel

[Kontinuation]

Fixes #3970 by making sure that we can always retrieve the widget manager owner from a session context, even if the session context does not have kernel attached. This gets rid of the exception when retrieving the kernel ID from session context with no kernel:

plugin.ts:153 Uncaught (in promise) TypeError: Cannot read properties of null (reading 'kernel')
    at getWidgetManagerOwner (plugin.ts:153:34)
    at async registerWidgetHandler (plugin.ts:175:25)
    at async plugin.ts:424:7

ipywidgets works properly after attaching ipykernel to a notebook that initially has no kernel after applying this fix.

[github-actions]

👈 Launch a binder notebook on branch Kontinuation/ipywidgets/fix-init-no-kernel