PhysicsAnalysis: fixed another bug in online conditional b-tag systematics - tagging info is always nominal

the btagsel tool uses the nominal set only and not varied for the systematic variations. Hence the decoration saved in BTagScaleFactorCalculator is only the nominal one while in ScaleFactorRetriever it was tried to access (non-existing) decorations for the systematic variations. This is fixed in the MR. Also added is a protection against extreme phase-space corners for the scale factors. Should the denominator in the computations get below epsilon (1e-3) the ratio is not trusted and a weight of 1 is used.