handling of cross-dc-folllower-count = 0

[vporyadke]

Introduced in #15974

When loading a follower group with RequireAllDataCenters = true, FollowerCountPerDataCenter = false, FollowerCount = 0, Hive makes an unsigned underflow, and converts in into a group with a giant FollowerCount

Such follower groups are present in export tables created by old versions of YDB (way before YDB was open-sourced)