It would be useful to allow segments to match visitors for which a given value is "NULL" (empty). For example, select all visitors that do not have a "custom variable 1" set.
Similarly, it would be useful to select all visitors, where a particular field (eg. "referrer keyword") is NOT NULL, ie. not empty nor NULL.
This feature is available by doing:
referrerKeyword== will select all visitors that don't have a keyword set, or
referrerKeyword!= will select all visitors that have a keyword set.
In d89a08b: Fixes #3932
Added documentation: Segment IS NULL / IS NOT NULL in piwik