[ntuple] Late model extension of untyped records broken

[jblomer]

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Description

• With buffered sinks, the new fields are attached to the zero field instead of the untyped record field
• Size and alignment of parent untyped record fields (for untyped record hierarchies) are not updated

Reproducer

See fix in #22507

ROOT version

master

Installation method

from sources

Operating system

n/a

Additional context

No response