Handle empty table when writing NanoAOD

[Dr15Jones]

PR description:

Avoid dereferencing a null pointer when writing an empty table.
This was uncovered by the UBSAN checks.

PR validation:

Code compiles.

[Dr15Jones]

front() returns a reference and should never be called if the container is empty.

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-37693/29515

• This PR adds an extra 12KB to repository

• There are other open Pull requests which might conflict with changes you have proposed:

  • File PhysicsTools/NanoAOD/plugins/TableOutputBranches.h modified in PR(s): Refactoring gen weight storage in EDM + Nano integration #32167

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for master.

It involves the following packages:

• PhysicsTools/NanoAOD (xpog)

@cmsbuild, @mariadalfonso, @gouskos, @fgolf can you please review it and eventually sign? Thanks.
@gpetruc, @swertz this is something you requested to watch as well.
@perrotta, @dpiparo, @qliphy you are the release manager for this.

cms-bot commands are listed here

[mariadalfonso]

please test

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5f57b7/24244/summary.html
COMMIT: f726ecd
CMSSW: CMSSW_12_4_X_2022-04-26-1100/slc7_amd64_gcc10
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/37693/24244/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

@slava77 comparisons for the following workflows were not done due to missing matrix map:

• /pool/condor/dir_222511/jenkins/workspace/compare-root-files-short-matrix/data/PR-5f57b7/39434.75_TTbar_14TeV+2026D88_HLT75e33+TTbar_14TeV_TuneCP5_GenSimHLBeamSpot14+DigiTrigger+RecoGlobal+HLT75e33+HARVESTGlobal

Summary:

• No significant changes to the logs found
• Reco comparison results: 2 differences found in the comparisons
• DQMHistoTests: Total files compared: 49
• DQMHistoTests: Total histograms compared: 3695434
• DQMHistoTests: Total failures: 8
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3695404
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 48 files compared)
• Checked 205 log files, 45 edm output root files, 49 DQM output files
• TriggerResults: no differences found

[mariadalfonso]

+xpog

technical PR

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @perrotta, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)

[qliphy]

+1