ISIS-565: honour NotPersistedFacet...

... when flushing EntityModel's collection of propertyScalarModels when the form is submitted.

What was happening here is that the contributed property 'lastInvoice' (in
Estatio) has no value on it, and also has no facet to handle it being
modified (it should not need one, since it declares itself to be not-persisted
using the NotPersistedFacet)