empty-qstringliteral: Make check work with Qt6 and ignore files from QML plugins
The QStringLiteral macro is very different in Qt5/Qt6, thus the preprocessor visitor is enabled to get the used Qt version.
In the generated files, like knewstuff/src/qtquick/.rcc/qmlcache/newstuffqmlplugin_qml/private/ErrorDisplayer_qml.cpp, Qt creates empty QStringLiterals. While Qt does not use empty QStringLiterals in the UI files anymore, it is not worth checking them though. Especially since we have the file name at hand already.