re-improve check on size of L1-uGT digis in HLTL1TSeed plugin

[missirol]

PR description:

This PR amends a mistake introduced in #40655. The check on size == 0 should not have been removed in #40655, mainly because, if BXVector::size() == 0, the call to BXVector::isEmpty(0) might lead to a crash (which is arguably a limit of the implementation of BXVector::isEmpty(int bx)).

Thanks to @swagata87 for indirectly reminding me of this.

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_0_X

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-41469/35331

• This PR adds an extra 20KB to repository

[cmsbuild]

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

It involves the following packages:

• HLTrigger/HLTfilters (hlt)

@cmsbuild, @missirol, @Martin-Grunewald can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @silviodonato 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-4ceb94/32261/summary.html
COMMIT: a6460e5
CMSSW: CMSSW_13_1_X_2023-04-30-0000/el8_amd64_gcc11
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmssw/41469/32261/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• No significant changes to the logs found
• Reco comparison results: 12 differences found in the comparisons
• DQMHistoTests: Total files compared: 48
• DQMHistoTests: Total histograms compared: 3460877
• DQMHistoTests: Total failures: 6
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3460849
• 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

@cms-sw/orp-l2 , please consider this PR for the upcoming IB (it is not as urgent as #41466, but I would prefer to have this PR integrated and backported soon).

[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