Fix Tab page provider params not saved in Designer

Steps to reproduce:
1. Select Nuxeo 10.10 + Web UI on Application Definition.
2. Create a Page Provider in Modeler:
2.1. Name it pp.
2.2 Add a simple predicate to dublincore:title.
3. Create a Tab (Listing) in Designer:
3.1. Name it tab.
3.2. Select pp as the provider.
3.3. Under "Show Advanced Options", set the value of dublincore_title (in the params field) as whatever.
3.4. After saving, notice that the value defined in the previous step was not persisted.

Additionally, when using Nuxeo 11.1, the predicates are not shown individually in the params field of the Tab editor, which reveals another issue: the field type is not being recognized as an object because it is in fact object | undefined | null.