Allow searching for dates which are unset

e60885aa changed the meaning of "Starts < 'today'" (and all other date
limits) to no longer include tickets whose dates are unset.  This
behavior change also made it imposible to search for such tickets, as
"Starts = '1970-01-01'" would also no longer find them, and the more
straightforward "Starts IS NULL" was invalid.

Allow IS NULL and IS NOT NULL searches on date ranges.
Add an UPGRADING note about the changed meaning of < in date limits
Add some simple tests for IS and IS NOT NULL
Allow IS/IS NOT with LimitDate, by providing DefaultEA for them

%DefaultEA only matters for the old Restrictions clauses; regardless,
permit the new format there, for consistency.
Merge branch '4.2/date-is-null' into 4.2-trunk