This change undoes a breaking change introduced by the 2.1 model
validation changes. Now an implementation of IValidableObject on a
top-level model will be called correctly with the 'empty' prefix instead
of the parameter name.
When fixing this we undid a workaround for another issue.
When validating a parameter that didn't bind we didn't correctly compute
the model name for 'fallback to empty prefix' cases.
