Check if we can make a Folder always have a sync engine

[erikjv]

During construction of a Folder object, a check (checkLocalPath) is done, and the object is only fully initialised when that succeeds. If it fails, the Folder is left incomplete (missing a sync engine, among things), and cannot really be used. This can lead to crashes (#11982) when the engine is accessed anyway.

We should check (and refactor) the Folder construction to be done in a static factory method that can either fail, or produce a valid object.

[DeepDiver1975]

that can either fail,

or creates a nop folder implementation - this could help to clean up the folder implementation with all the condition checking if _engnie exists of not ...