cubicweb #2033165 is_instance doesn't check for None value when a row is specified [resolved]

this may lead to unobvious error with outer join

prioritynormal
typebug
done in3.13.9
load0.100
load left0.000
closed by#77ed9b2fc9a5 [selectors] EClassSelector (eg is_instance & all) should check for None value (possible w/ outer join) when a row is specified. Closes #2033165