- Fixed bug where it tried to sum columns with a count aggregate -- only sum if sum or average- Export empty filter vars, anyway, to allow for null filters.
Changed list builder so it can now work with filter values which is the empty string, and provide a special label and where clause for such null values. Used in bug-tracker's patch list (and soon also in bug-tracker)
Changed export_vars so empty values are not suppressed after all if they're specified directly in the var-spec { varname value }. Also added a test case for export_vars