[sql generation] fix missing join with outer joins w/ rhs invariant variable...
... where the principal where not the processed relation.
This led to major refactoring in outer join handling, which is now much
clearer and powerful.
Closes #1579228
[test, cw props] ensure we can't add user specific / system-wide properties
we had this pb on an historical instance in our intranet (eg. running
for about 8 years).