Denormalizer step: 100 + null = null

Using sum function for denormalizer step with null and not-null input values hadles cases for
100 + null = null
Compare to GroupBy step which handle such cases as
100 + null = 100

This is caused by Denormalizer step uses ValueDataUtil.plus( ... ) calls,
while GroupBy uses ValueDataUtil.sum( ... ) calls.

see
https://github.com/pentaho/pentaho-kettle/blob/3542bec0507e899bc16fee9a7bf40ed43b09f0fc/core/src/org/pentaho/di/core/row/ValueDataUtil.java#L401

and
https://github.com/pentaho/pentaho-kettle/blob/3542bec0507e899bc16fee9a7bf40ed43b09f0fc/core/src/org/pentaho/di/core/row/ValueDataUtil.java#L451

implementation details.