Fix selectivity clear action on single value

When clearing a selectivity widget with the little arrow, it set the value to null. This is not a pbm in most cases.

However, when you design a search with a nuxeo-user-suggestion bound to a named parameter, clicking reset results in an null param value. As a direct result the search does not return any result. For instance the request sent for a nuxeo-user-suggestion bound to a dc_creator named param is:

http://localhost:5000/nuxeo/api/v1/search/pp/default_search/execute?currentPageIndex=0&offset=0&pageSize=40&sortBy=dc%3Acreated&sortOrder=asc&dc_creator=null&quickFilters=

Note the

dc_creator=null

clause that makes the query return no result.