AdvancedSearchController bug doesn't allow independent usage of #getSelectedDocuments and #saveCollection in combination with "Select all"

Steps to reproduce:

1. Implement a custom advanced search workflow that makes use of both searcher.getSelectedDocuments and searcher.saveCollection (NOTE: the order in which the methods will be invoked will have a different result)
2. Select "Select all" in the advanced search add-on
3. Perform custom workflow operation

Expected: the workflow in both method invocations will have the correct number of selected items

Outcome: in the second method invocation the total number of selected items will be 0

If we invoke first getSelectedDocuments and then saveCollection the result will be that the property hippocollection:uuids at /collections/admin/admin will be an empty multi-string property while we clearly have documents selected.

If we invoke first saveCollection and then getSelectedDocuments the iterator returned although it has the total size of the selected documents the hasNext or next methods will return false & null respectively.