Return 0 if $sum is null (aggregation framework)

So I'm using aggregation framework to sum some results in given time blocks. My question is how do I return count: 0 every time if there was no results in the time block.

So let's say in first ten seconds my count: 7 in next ten seconds count: 2 and then if in the third time block there was nothing I would like aggregate function to return count: 0

Below simple summing operation:

$group: {
  _id: 1,
  count: { $sum: 1 }
}

I have tried using $ifNull, but it didn't work for me:

$group: {
  _id: 1,
  count: { $sum: { $ifNull: [ $count, 0 ] } }
}