Fix HepMC3 conversion on MC without event weights (particle guns)

[mseidel42]

PR description:

• Resolves Updates for Rivet 4 #44603 (comment) by adding dummy event weights if none are present (most likely for particle guns)
• Add test to avoid this problem in the future

PR validation:

% runTheMatrix.py -l 11601.0 --ibeos -t 4
...
1 1 1 1 1 tests passed, 0 0 0 0 0 failed

[cmsbuild]

cms-bot internal usage

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-45367/40795

• This PR adds an extra 20KB to repository

[cmsbuild]

A new Pull Request was created by @mseidel42 for master.

It involves the following packages:

• GeneratorInterface/RivetInterface (generators)

@GurpreetSinghChahal, @SiewYan, @alberto-sanchez, @bbilin, @cmsbuild, @menglu21, @mkirsano can you please review it and eventually sign? Thanks.
@alberto-sanchez, @mkirsano this is something you requested to watch as well.
@antoniovilela, @rappoccio, @sextonkennedy you are the release manager for this.

cms-bot commands are listed here

[smuzaffar]

please test workflow 135.2,1317.0,2017.2,10001.0,10801.0,11604.0

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-a52fd9/40199/summary.html
COMMIT: f38a6d9
CMSSW: CMSSW_14_1_X_2024-07-02-2300/el8_amd64_gcc12
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/45367/40199/install.sh to create a dev area with all the needed externals and cmssw changes.

• DAS Queries: The DAS query tests failed, see the summary page for details.

Comparison Summary

There are some workflows for which there are errors in the baseline:
10001.0 step 6
10801.0 step 6
11604.0 step 3
1317.0 step 5
135.2 step 4
2017.2 step 4
The results for the comparisons for these workflows could be incomplete
This means most likely that the IB is having errors in the relvals.The error does NOT come from this pull request

Summary:

• You potentially removed 34 lines from the logs
• ROOTFileChecks: Some differences in event products or their sizes found
• Reco comparison results: 6 differences found in the comparisons
• DQMHistoTests: Total files compared: 53
• DQMHistoTests: Total histograms compared: 3726940
• DQMHistoTests: Total failures: 12
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3726906
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 52 files compared)
• Checked 230 log files, 191 edm output root files, 53 DQM output files
• TriggerResults: no differences found

[smuzaffar]

urgent

@cms-sw/generators-l2 , can you please review this? This should fix many failing relvals

[antoniovilela]

+1

• Will be merged once @cms-sw/generators-l2 sign.

[antoniovilela]

merge