Fix sherpa weights when no weight list is provided

[thuer]

Fix the sherpa event weights in case no reordering of weights is applied (the deletion of the original weight vector should only be done if we want to use the new vector)

[cmsbuild]

A new Pull Request was created by @thuer (Sebastian Thüer) for CMSSW_8_1_X.

It involves the following packages:

GeneratorInterface/SherpaInterface

@cmsbuild, @govoni, @perrozzi, @thuer, @davidlange6 can you please review it and eventually sign? Thanks.
@agrohsje, @mkirsano this is something you requested to watch as well.
@slava77, @smuzaffar you are the release manager for this.

cms-bot commands are list here #13028

[perrozzi]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/14964/console

[cmsbuild]

+1
Tested at: 6577463
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-15733/14964/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-15733/14964/summary.html

[perrozzi]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_1_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @smuzaffar

[davidlange6]

+1