CMake: fix Qt6 build on MacOS (partial fix)

Before this fix, unset variables lead to the following cmake configuration failure:

-- Found QT6
-- Using QT6 (6.2.1)
CMake Error at CMakeLists.txt:150 (if):
  if given arguments:

    "STREQUAL" "5.8.0" "OR" "STREQUAL" "5.7.1"

  Unknown arguments specified

-- Configuring incomplete, errors occurred!

Note that even with this