The behavior is now that filters on "null" will match rows with no
values. The behavior in the past was inconsistent; sometimes these
filters would match and sometimes they wouldn't.
Adds tests for this behavior to SelectorFilterTest and
BoundFilterTest, for query-level filters and filtered aggregates.
Fixes#2750.
- Add central doc for multi-value dimensions, with some content from other docs.
- Link to multi-value dimension doc from topN and groupBy docs.
- Fixes a broken link from dimensionspecs.md, which was presciently already
linking to this nonexistent doc.
- Resolve inconsistent naming in docs & code (sometimes "multi-valued", sometimes
"multi-value") in favor of "multi-value".
Jonathan Wei
David Lim
Vadim Ogievetsky
Erik Dubbelboer
Kirill Kozlov
Gian Merlino