uninformative error message on missing cmake_minimum_required in user projects

Suppose a user creates a Qt-based project with a CMakeLists.txt that is missing a call to cmake_minimum_required(VERSION 3.17) or higher.

Then a call like find_package(Qt6 COMPONENTS Core) will produce the following error

CMake Error at /home/jobor/Qt/dev-cmake/lib/cmake/Qt6/Qt6Dependencies.cmake:14 (list):
  list index: 3 out of range (-2, 1)
Call Stack (most recent call first):
  /home/jobor/Qt/dev-cmake/lib/cmake/Qt6/Qt6Config.cmake:69 (include)
  CMakeLists.txt:4 (find_package)

instead of something more enlightening like "CMake version so-and-so required".

Solution: make sure that we do not access list elements beyond the list's end in the exported files.
The exported cmake files should work without setting policies / requiring any newer CMake features.