Asset indexes: Search in subfolders option should be visible when there is an active filter, even with empty search field

[thupsi]

When filtering asset indexes it would be very useful to enable the search in subfolders option, even if the search field is empty.

Example use case: The user wants to find all assets related to an entry. The assets may exist in nested folders, and to trigger the subfolder search, with current functionality, the user has to type something in the search field, in addition to activating the relation filter.