Handle the exceptional case of two non-empty account/standard-folder sets producing an empty intersection.
Handle the exceptional case of two non-empty account/standard-folder
sets producing an empty intersection.

Which means non-matching (normally an empty set means match all).

enthought/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility