Removing valuelist provider configuration breaks the radio group plugin

This should fix an annoying developer configuration problem. Reproduction path for the Radio group plugin:

• Add a Radio group field to a document type (using the document type editor)
• Add a valid source: the items show up as radio buttons in the document prototype view
• Add a value in the valuelist provider property for the field (e.g. 'blabla': the items disappear because the provider cannot be found).
• Remove this value again. Result: the items do not appear anymore.

Cause of this: the configuration property on the cluster.options field is created and is now empty. The empty (so not null) value is used to try to lookup a value list provider, which fails. You have to remove the empty string property to get the radio button values to show up again.

The fix would be to check on "blank or null" instead of only on null to decide to use the default for this property.

Note: this same behavior may be used for other configuration properties of the Selection plugin field types as well. And thus could be fixed there also.