PDI-10250 - "Group by" step - Minimum aggregation doesn't work

Fix provided.
2 new variables introduced: KETTLE_AGGREGATION_MIN_NULL_IS_VALUED and
KETTLE_AGGREGATION_ALL_NULLS_ARE_ZERO.
junits added.
PDI-6960 - Group by and Memory group by ignores NULL values ("sum" aggregation)

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.
stash PDI-9662