refactor(core): Allow null values in a variant
refactor(core): Allow null values in a variant

JGrothoff/open62541,jpfr/open62541,jpfr/open62541,jpfr/open62541,JGrothoff/open62541,JGrothoff/open62541,JGrothoff/open62541,open62541/open62541,open62541/open62541,open62541/open62541,open62541/open62541,jpfr/open62541