presto-main/src/main/java/io/prestosql/operator/scalar/ScalarFunctionImplementation.java/ScalarFunctionImplementation/getNullConvention
class ScalarFunctionImplementation:        public NullConvention getNullConvention()
        {
            checkState(getArgumentType() == VALUE_TYPE, "nullConvention only applies to value type argument");
            return nullConvention.get();
        }