[io] don't miss writing a histogram that is only in a last file with option -n 2

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://github.com/root-project/root/issues/9022

Explores solution suggested by @jblomer

fyi @will-cern

Checklist:

• [x] tested changes locally
• [x] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    21 files      21 suites   3d 8h 59m 8s ⏱️  3 220 tests  3 220 ✅ 0 💤 0 ❌ 65 897 runs  65 897 ✅ 0 💤 0 ❌

Results for commit 989b730f.

:recycle: This comment has been updated with latest results.

[ferdymercury]

@hageboeck your test looks green now ;)