[ntuple] Incorrect treatment of I/O rule with empty source

[jblomer]

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Description

I/O customization rules may have an empty list of source fields. In this case, RNTuple should not try to create a staging area and not request a staging class, which may not exist.

Reproducer

cms-sw/cmssw#50744

ROOT version

master

Installation method

from sources

Operating system

n/a

Additional context

No response

[github-actions]

Hi @dpiparo, @jblomer,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖