windeployqt: Fall back comparing names of QML modules when their class name is empty

QtWebEngine and some other modules have empty class names, so they are
dropped when concatenating QML imports.

Task-number: QTBUG-61957
Change-Id: I06ccf8ab54b2d3ffba04f15746722f5b3596d5d9
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>