Deal with weights for HepMC3

Deal with weights for HepMC3: prevent exceptions in case the weights are not set.

Tag @jchapman

This merge request affects 2 packages:

• Event/xAOD/xAODTruthCnv
• Generators/GeneratorObjectsTPCnv

Affected files list will not be printed in this case

Adding @akraszna as watcher

added EDM Generators master review-pending-level-1 labels

CI Result SUCCESS (hash 9f5fbc5a)

	Athena	AthSimulation	AthGeneration	AnalysisBase	AthAnalysis	DetCommon
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthAnalysis: number of compilation errors 0, warnings 0
DetCommon: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 30931]

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 7c528bd1

added sweep:ignore label