[DF] Also check if fOutputFiles is empty before warning untriggered snapshot

[karuboniru]

Fixes #14132

This Pull request:

Changes or fixes:

Also check if fOutputFiles is empty before giving warning "A lazy Snapshot action was booked but never triggered".

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #14132 (the other issue with broken gDirectory seems to have gone in latest ROOT (6.32.06), so can consider as this bug is fixed)

[dpiparo]

Thanks for this PR. I would like to ask a question about your comment above. Do you mean that in 6.32.06, the reported breakage of gDirectory is fixed and only warnings are left to be fixed (by this PR)?

[github-actions]

Test Results

    14 files      14 suites   3d 9h 8m 12s ⏱️
 2 699 tests  2 699 ✅ 0 💤 0 ❌
35 532 runs  35 532 ✅ 0 💤 0 ❌

Results for commit bced511.

♻️ This comment has been updated with latest results.

[karuboniru]

@dpiparo exactly, I tested with the reproducer in the issue with 6.32.06 and the file saving function is back, this might have been fixed in previous version.

[dpiparo]

Starting from your commit, I added a test case in order to check in ROOT's battery of tests also the output when a lazy snapshot is triggered in MT mode. This was missing, only the case which was there was the one when a lazy snapshot was not triggered in MT mode.