Fix folder dialog always opens even if it is set and not empty
The original code detects if the folder is empty. But the problem is: file loading is in signal/slot mode, it cannot immediately return files. So the count()
is always empty when you boot JuK.
In this patch, I changed the detection rule. If users haven't added any folders, open the dialog to add one. If the user has added only empty folders, then what they need to do is to put music files in their folders, not adding more folders, so folder dialog won't open in this situation.
This PR should fix bug https://bugs.kde.org/show_bug.cgi?id=409936
This PR is for master branch. For Applications/19.08 branch, check !11 (merged)