Root object is inaccessible from metabase if it contains split fields

[cthulhu-rider]

• see Complex object is completely unreadable after upgrade to SDK RC-9 #2448

currently, root objects has no split fields. In theory, they may have (multi split chain?). Metabase accepts such objects carried by the last child or the linking one. Then metabase fails Exists op with no split info on parent object error (object couldn't be read from the engine/node). This behavior is incorrect. Possible corrections:

• deny storage of such objects
• accept objects but don't fail like now

[roman-khimov]

I think we better forget tree-like splits for now and just deny these. Practically, there is no need for them once we solve nspcc-dev/neofs-api#263.