Bugfix: raise error when running a validation with no runs

[hroskes]

Previously, it would instead run anyway, including all runs in the dataset

backport of #12925

[cmsbuild]

A new Pull Request was created by @hroskes (Heshy Roskes) for CMSSW_7_6_X.

It involves the following packages:

Alignment/OfflineValidation

@diguida, @cerminar, @cmsbuild, @franzoni, @mmusich, @davidlange6 can you please review it and eventually sign? Thanks.
@mschrode, @ghellwig, @mmusich, @tocheng, @tlampen this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

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[mmusich]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/10530/console

[cmsbuild]

+1
Tested at: 4c9f6b2
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12926/10530/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12926/10530/summary.html

[mmusich]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_6_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1