improve how HLTPrescaler checks presence of (Stage-2) L1-uGT results

[missirol]

PR description:

This PR improves how the HLTPrescaler checks for the availability of the Stage-2 L1-uGT results of BX=0 before accessing them to find the index of the prescale column.

This check is currently done by first calling BXVector::begin(0), but the latter call can in principle result in a runtime error if the content of the BXVector is empty, see here.

The check is updated using BXVector::isEmpty(0), which shouldn't lead to runtime crashes under any circumstances provided #41572 and its backports are integrated.

Merely technical. No changes expected.

PR validation:

None.

If this PR is a backport, please specify the original PR and why you need to backport that PR. If this PR will be backported, please specify to which release cycle the backport is meant for:

CMSSW_13_1_X
CMSSW_13_0_X

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-41575/35444

• This PR adds an extra 12KB to repository

[cmsbuild]

A new Pull Request was created by @missirol (Marino Missiroli) for master.

It involves the following packages:

• HLTrigger/HLTcore (hlt)

@cmsbuild, @missirol, @Martin-Grunewald can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @silviodonato, @fwyzard this is something you requested to watch as well.
@perrotta, @dpiparo, @rappoccio you are the release manager for this.

cms-bot commands are listed here

[missirol]

please test

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-0ed72e/32449/summary.html
COMMIT: b00b191
CMSSW: CMSSW_13_2_X_2023-05-07-0000/el8_amd64_gcc11
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/41575/32449/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• You potentially added 4 lines to the logs
• Reco comparison results: 12 differences found in the comparisons
• DQMHistoTests: Total files compared: 48
• DQMHistoTests: Total histograms compared: 3460836
• DQMHistoTests: Total failures: 6
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3460808
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 47 files compared)
• Checked 207 log files, 159 edm output root files, 48 DQM output files
• TriggerResults: no differences found

[missirol]

+hlt

[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, @rappoccio (and backports should be raised in the release meeting by the corresponding L2)

[rappoccio]

+1