useDefaultValueForNull need to be explicitly set to true starting druid 28

[anton-antropov]

This issue took up half my day, so I'm sharing it with the community. Starting from Druid 28, the useDefaultValueForNull is set to false, affecting aggregations. For example, in my case:

$main.sum($c1) + $main.sum($c2)

was returning null even though one of the values was non-null.

setting useDefaultValueForNull explicitly to true solved the issue