[FileMetaDataProvider] Show item count also for empty directories

Summary:
In case only one dir is selected, its item count is show, both for empty
and non-empty directories. When multiple directories are selected, the
summed count was only shown when at least one was non-empty.

Test Plan:
1. Select one empty directory - "Size: 0 items"
2. Select multiple non-empty directories - "Total Size: xxx items"
3a. Old: Select multiple empty directories - "Total Size" not shown
3b. New: Select multiple empty directories - "Total Size: 0 items"

Reviewers: #baloo, #dolphin, nicolasfella, astippich, ngraham, elvisangelaccio

Reviewed By: #baloo, #dolphin, ngraham

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20910