[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
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html