Revisit dynamic manifest sender and update messages to ideally avoid nil manifests compeltely

[masih]

Now that there is a dedicated flag for pausing F3 participation via manifest the value of manifest itself should never be nil. But in code it is technically possible to propagate a nil manifest.

Refactor code such that:

• it remains resilient against nil manifest, i.e. fails gracefully.
• API disallows propagation of nil manifest, i.e. fails early.