[applets/taskmanager] Fix showing window thumbnails

windows is of type QVariantList, which since Qt 6.7 is not considered an array any more, so this will always be false

See https://code.qt.io/cgit/qt/qtdeclarative.git/commit/?id=b9bfdea0e2c6721d2306af0ecc44f88da9988957

BUG: 479050


(cherry picked from commit 4648d3b9)