Fix photo widget validation

[harminius]

Fixes https://github.com/MerginMaps/server-private/issues/3230

Core issue: KeyErrorin validations when Data Driven Override is FieldBasedProperty. When Data driven override is enabled we assume there has to be an expression. However, it is also possible to set field based override.

Field: 'Thumbnail_link'
------------------------------
{'DocumentViewer': 2,
 'DocumentViewerHeight': 0,
 'DocumentViewerWidth': 0,
 'FileWidget': False,
 'FileWidgetButton': True,
 'FileWidgetFilter': '',
 'FullUrl': True,
 'PropertyCollection': {'name': NULL,
                        'properties': {'propertyRootPath': {'active': True,
                                                            'expression': '"Thumbnail_link"',
                                                            'type': 3},
                                       'storageUrl': {'active': True,
                                                      'field': 'Thumbnail_link',
                                                      'type': 2}},
                        'type': 'collection'},
 'RelativeStorage': 0,
 'StorageAuthConfigId': NULL,
 'StorageMode': 0,
 'StorageType': NULL,
 'UseLink': True}