Add NULL support to rollup(hyperloglog)

[jerryxwu]

Is your feature request related to a problem? Please describe.
Add NULL support to rollup(hyperloglog) to improve user experience.

Describe the solution you'd like
rollup(hyperloglog) should simply ignore NULL hyperloglog objects instead of returning an error. For Toolkit 1.12.0, the returned error is:

ERROR: other_ is null
CONTEXT: extension/src/hyperloglog.rs:277:5

Consider this query:

SELECT distinct_count(rollup(logs))
FROM (
	(SELECT hyperloglog(16, v::text) logs FROM generate_series(1, 5) v)
	UNION ALL
	(SELECT hyperloglog(16, v::text) FROM generate_series(6, 10) v WHERE v <=5)
) hll;

It should return the same value as this query as the second hyperloglog object will be NULL:

SELECT distinct_count(rollup(logs))
FROM (
	(SELECT hyperloglog(16, v::text) logs FROM generate_series(1, 5) v)
) hll;

Describe alternatives you've considered
An alternative is to do nothing and ask the user to always include FILTER (WHERE hll IS NOT NULL) clause if hll can be null. For the example above, the query would look like this,

SELECT distinct_count(rollup(logs) FILTER (WHERE logs IS NOT NULL))
FROM (
	(SELECT hyperloglog(16, v::text) logs FROM generate_series(1, 5) v)
	UNION ALL
	(SELECT hyperloglog(16, v::text) FROM generate_series(6, 10) v WHERE v <=5)
) hll;

The alternative works, it's just more boilerplate SQL code to write.