cubicweb #2841199 fix crash on db-dump -f portable [validation pending]

it's looking for 'identity_relation' table while identity is a virtual relation which has no associated table

prioritynormal
typebug
done in3.16.4
load0.100
load left0.000
closed by#7652c3d46ba3 [portable dump] skip virtual relations, they have no table associated. Closes #2841199