PDI-10250, PDI-6960 - ["Group by" step - Minimum aggregation doesn't work], [Group by and Memory group by ignores NULL values ("sum" aggregation)]

[PDI-10250]
Fix provided.
2 new variables introduced: KETTLE_AGGREGATION_MIN_NULL_IS_VALUED and
KETTLE_AGGREGATION_ALL_NULLS_ARE_ZERO.
junits added.

[PDI-6960]
Do not fill with 0 by default new aggregations.
If aggregation still null - this will return null or 0 depends on 'KETTLE_AGGREGATION_ALL_NULLS_ARE_ZERO' variable.
Junits added.
PDI-11530 - "Memory Group by" step - Minimum aggregation doesn't work

fix added.
junit tests added.
PDI-9662, PDI-11597 - [Denormalizer step: Minimum aggregation doesnt' work], [Denormalizer step: 100 + null = null]

[PDI-9662]
fix.
Avoid newGroup() intialize min with null.
Added support of new varibles to handle aggregation null-cases.

[PDI-11597]
fix.
replace ValueDataUtil.plus(...) call with sum(...) to handle nulls
properly.
Merge pull request #440 from prakapenka/PDI-10250

PDI-10250,6960,11530,9662,11597 cummulative fix