changed SampleTests to comply with parentEvent being nullable, delete via parent event now works
changed SampleTests to comply with parentEvent being nullable, delete via parent event now works

PhenotypeFoundation/PhenotypeDatabase-legacy,NetherlandsMetabolomicsCentre/GSCF,PhenotypeFoundation/PhenotypeDatabase-legacy,thehyve/GSCF,TNO/PhenotypeDatabase,thehyve/GSCF,TNO/PhenotypeDatabase,PhenotypeFoundation/PhenotypeDatabase,NetherlandsMetabolomicsCentre/GSCF,PhenotypeFoundation/PhenotypeDatabase-legacy,PhenotypeFoundation/PhenotypeDatabase