Need way to count points where condition is true

I think we need a way to count the number of nodes where a certain condition is true.
Currently the only way to count things is with the InfluxQL `Count()` node. However this counts where the given field is non-null. Meaning to count where a condition is true, you have to do `|where(lambda: foo)|count("arbitrary_field")`. The problem with this is that if the `where()` drop all the points, then `count` doesn't receive anything, and thus doesn't emit anything either. So you can never get a count of 0.

Start by examining the existing InfluxQL Count() node and how it receives output from where(). Confirm how the current pipeline behaves when where() drops every point; done should provide a conditional count that emits 0 when no points satisfy the condition.

Written by the indexing model from the issue text.