qt6_generate_foreign_qml_types fails if there are no QML elements declared

qt_generate_foreign_qml_types should "Register[s] types from one target in a QML module.".

However, it also checks whether the target contains QML elements at all, and fails if it doesn't.

If you have a list of targets that might or might not contain QML_ELEMENTs or QML_NAMED_ELEMENTs, this is very annoying. Currently some of my targets only declare classes whose instances are added to the  QML engine by C++ code in our main function, but they may get QML_ELEMENTs any time.