cubicweb-wsme #5292808 when GETing an entity that does not exist, we should obtain an empty json list [validation pending]

(rather than plain html)

prioritynormal
typebug
done in0.2.1
load0.200
load left0.000
closed by#6dead8767f45 Returns proper content with entity-related 404 errors
patchReturns proper content with entity-related 404 errors [applied]Return None when GETting a non-existing entity [reviewed]