cubicweb #2207180 rql rewrite bug w/ outer join and multiple expressions [validation pending]

the expressions aren't properly ORed.

prioritynormal
typebug
done in3.14.5
load0.100
load left0.000
closed by#a4b009ba92ce [rql rewrite] when a subquery has to be introduced, properly return the snippet expression so that further expressions are properly ored. Closes #2207180