[Lens] Offer option to drop 0 values generated by count and sum aggs

[flash1293]

Currently some charts behave confusing because the data table passed to the chart does not consistently pass "no data" as a null value in the cell (in case of count or sum, a zero is filled in). To fix this, the agg configs of sum and count have to be extended with an option to turn 0 into null in case there are no documents.

In Lens, a migration has to turn on this flag in the "advanced settings" accordion in the dimension editor, for newly created columns it should be turned off. This will pass the responsibility to handle gaps in the data in a meaningful way to the chart which can handle them in a better way.

For count and sum columns used in formula this switch should be always set to true as it's not possible to use the resulting value in calculations otherwise.

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)