Geant4PrimaryHandling: fix issue with multiple vertices in Geant4 GeneralParticleSource #1213

Job	Run time
Upload Event Payload	2s
default (LCG_100/x86_64-centos7-gcc10-opt)	38m 43s
default (LCG_101/x86_64-centos7-gcc11-opt)	24m 17s
default (LCG_102/x86_64-centos9-gcc11-opt)	37m 46s
g4units (dev3/x86_64-centos7-gcc11-opt)	1h 23m 23s
key4hep (/cvmfs/sw-nightlies.hsf.org/key4hep/)	58m 1s
default (LCG_102/x86_64-centos7-clang12-opt)	39m 18s
non-shared (LCG_102/x86_64-centos7-gcc11-opt)	12m 40s
default (LCG_102/x86_64-ubuntu2004-gcc9-opt)	24m 45s
default (dev3/x86_64-centos7-gcc11-opt)	1h 26m 5s
default (dev4/x86_64-centos7-gcc11-opt)	1h 24m 22s
	8h 9m 22s

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