Fix windows build in case of empty CMAKE_PREFIX_PATH

[christopho]

Fix build failure if CMAKE_PREFIX_PATH is empty:

-- Found Solarus: /builds/solarus-games/solarus-quest-editor/staging/include (found suitable exact version "2.0.0")
-- Found SolarusGui: /builds/solarus-games/solarus-quest-editor/staging/include
CMake Error at build/_deps/qlementine-src/CMakeLists.txt:22 (string):
  string sub-command FIND requires 3 or 4 parameters.


-- Configuring incomplete, errors occurred!
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

[oclero]

Thank you!