V0 monitoring: fix normalization for empty plots

[mtosi]

fix of the nan reported by @slava77 in
#15090

[cmsbuild]

A new Pull Request was created by @mtosi (mia tosi) for CMSSW_8_1_X.

It involves the following packages:

DQM/TrackingMonitorClient

@cmsbuild, @dmitrijus, @vanbesien, @davidlange6 can you please review it and eventually sign? Thanks.
@makortel, @idebruyn, @threus, @fioriNTU, @hdelanno 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

[dmitrijus]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/14681/console

[cmsbuild]

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

[cmsbuild]

+1
Tested at: 86e174e
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-15531/14681/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-15531/14681/summary.html