cubicweb-container #2708808 adapter: .parent computation crashes if there is no parent [resolved]

Instead of just returning None.

priorityimportant
typebug
done in1.0.1
load0.100
load left0.000
closed by#6796439dbe83 [entities] if there is no parent, return None instead of crashing (closes #2708808)
patchentities: if there is no parent, return None instead of crashing (closes #2708808) [applied]