Fix global Jvt SF retrieval in JetJvtEfficiency

With the refactoring of the Jvt-related analysis tools in !64846 (merged), the "dummy value" of the Jvt SF for jets outside the validity range was changed from 1.0 to -1.0 This wasn't propagated to functionality of JetJvtEfficiency to return a Jvt SF for the whole event. This yields to negative total Jvt SFs in some events. Although the JetJvtEfficiency is legacy code and users should move to the CPalgs or the refactored set of Jvt tools, we decided to fix this for people still in the migration phase.

Tagging @robouque who proposed this as well as @gotero and @aoneill for MET/PU-tagging.