JcrPropertyValueModel must expose NULL_DATE

As part of CMS-5576, the constant o.h.f.m.PropertyValueProvider#NULL_DATE was introduced. This NULL_DATE is stored as part of model in the repository. o.h.f.m.p.JcrPropertyValueModel#getObject was also adjusted to convert NULL_DATE to null in case it was retrieved. The other code paths did not contain such a conversion, leading issues such as CMS-10227.

For 11.1 onwards we decided to fix this by removing the conversion of NULL_DATE from o.h.f.m.p.JcrPropertyValueModel. This makes the behavior regarding dates more in line with other primitive types that have an "empty” value such as strings. NULL_DATE is a “normal” value that can be set and retrieved, representing an empty date.

Hippo provided plugins that edit date properties will use an additional model that converts NULL_DATE to and from null when needed. Custom plugins can use this model as well, or implement some custom code to handle the NULL_DATE value if needed.