[75X] Fix crash in pat::MET when no uncertainties are added

[blinkseb]

Backport of #11394 for 7.5.x

[cmsbuild]

A new Pull Request was created by @blinkseb (Sébastien Brochet) for CMSSW_7_5_X.

[75X] Fix crash in pat::MET when no uncertainties are added

It involves the following packages:

DataFormats/PatCandidates

@cmsbuild, @vadler, @monttj can you please review it and eventually sign? Thanks.
@gpetruc this is something you requested to watch as well.
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@Degano you are the release manager for this.
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