Replace unnecessary uses of UNKNOWN_NULL_COUNT

[vyasr]

Description

This PR replaces uses of cudf::UNKNOWN_NULL_COUNT where the null count is either already known or trivially computed.

Contributes to #11968

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

It'd be nice to know if we're eliminating many "real-world" calls to the null_count kernel by supplying these values. Not sure how to measure that well, but perhaps benchmarks would show it?

[vyasr]

It'd be nice to know if we're eliminating many "real-world" calls to the null_count kernel by supplying these values. Not sure how to measure that well, but perhaps benchmarks would show it?

I agree on both counts, that is an interesting question, and one that is hard to answer. A good starting point would be defining what really constitutes a real-world use case. Also, the changes in this PR come in two flavors: 1) precomputing a null count, in which case we're doing more work up front under the assumption that it will eventually be necessary, and 2) propagating a known null count, which is a strict reduction in work. The latter is more common in this PR and is the case where we'd hope for kernel reduction of course, but depending on the workflow being benchmarked that affected might be washed out by changes of the first type in instances where the null count is not actually used. Hard to say without asking a pretty precise question I suspect.

[vyasr]

/merge