QComboBox's currentIndex() changes from -1 to 0 when adding data to an empty model

The issue appears to be this change: https://code.qt.io/cgit/qt/qtbase.git/commit/src/widgets/widgets/qcombobox.cpp?id=a4ea0d9eacd574a6a96f70b138dcb111e9d11d21

Before that change, after a model was set, `currentIndex()` would be set to -1 and an editable combobox would not have any text in the edit line. After that change, setting the model would cause `currentIndex()` to be set to 0 and the line edit to be filled with the first entry from the model.

This changes the behavior of existing applications. See https://bugs.kde.org/show_bug.cgi?id=426800 for an example.