Add 'Empty' option in Notifications Custom Filters LiveData drop-downs to ensure consistency with other LiveData tables

Currently, on Notifications Custom Filters Live Data, all filter boxes that have drop-downs (thus their value can be selected by mouse) do not have an 'empty' value in order to display back all values under that filter by mouse.

The user needs to clear manually the content of the filter using the keyboard in order to have all entries displayed back in the table, which is not so intuitive in all cases.

As the Attachments LiveData has for example, on filters that have drop-downs should be available an 'empty' option in the drop-downs, in order to display back all entries using the mouse.