Update metadata.py

This would fix plone/plone.app.vocabularies#59, but I'm only floating this change as an idea, hoping for some feedback.

`z3c.form.widget.Widget.update()` passes the result of `creatorsDefault` (which is called by an `IValue` adapter's `get()` method) to `plone.app.z3cform.converters.AjaxSelectWidgetConverter.toWidgetValue()`.

`getSecurityManager().getUser()` returns a regular `<PropertiedUser>` for most requests and thus the return value is a tuple, e.g. `('admin',)`.

However, for ajax requests, e.g. `/++add++MyContentType/++widget++form.widgets.IMyBehavior.behavior_field`, the user is anonymous:

```
(Pdb) user
<SpecialUser 'Anonymous User'>
(Pdb) pp user.__dict__
{'__': '', 'domains': [], 'name': 'Anonymous User', 'roles': ('Anonymous',)}
```

Thus, the return value is the tuple `(None,)`.  This breaks `AjaxSelectWidgets` when the content type includes the Dublin Core behavior or just the ownership field, even though the field itself is a `RelationList` with a `value_type=RelationChoice( source=CatalogSource(portal_type='MyContentType) )`.  In other words, even though the broken field has no need for the item's creator, the creator gets calculated anyway when the request is handled.

The problem with the tuple `(None,)` is that `toWidgetValue()` does not know how to deal with it and so it returns `None` instead of `field.missing_value` as it should.

This could be fixed in at least four different places:

1. plone.app.dexterity.behaviors.metadata.creatorsDefault
2. plone.app.z3cform.converters.AjaxSelectWidgetConverter.toWidgetValue
3. z3c.form.widget.Widget.update
4. have getSecurityManager().getUser() return the logged in user even on Ajax requests.

1. is the origin of the unexpected value, however, I don't know if making it return `None` for anonymous users would make any other code mad.
2. is the place that does not expect a `(None,)` tuple instead of `None`, so this could be fixed by simply adding a check for `(None,)`.
3. is the "middleman" between 1 and 2, so it could convert `(None,)` to `None` before passing it on.
4. I have no idea what this would entail.

plone/app/dexterity/behaviors/metadata.py

@@ -246,7 +246,7 @@ class IOwnership(model.Schema):
 def creatorsDefault(data):
     user = getSecurityManager().getUser()
     # NB: CMF users are UTF-8 encoded bytes, decode them before inserting
-    return user and (safe_unicode(user.getId()),)
+    return user and user.getId() and (safe_unicode(user.getId()),)
 
 CreatorsDefaultValue = ComputedWidgetAttribute(
     creatorsDefault,