cmake: Map cmake None to meson empty (hyprwm#3614)

While not explicitly mentioned by the cmake documentation, cmake
upstream seems to think of None as a vaild CMAKE_BUILD_TYPE. [1]
Handle it properly by mapping it to meson's empty.

[1]: Kitware/CMake@ce1cadd

CMakeLists.txt

@@ -44,6 +44,8 @@ if(CMAKE_BUILD_TYPE)
         set(BUILDTYPE_LOWER "debugoptimized")
     elseif(BUILDTYPE_LOWER STREQUAL "minsizerel")
         set(BUILDTYPE_LOWER "minsize")
+    elseif(BUILDTYPE_LOWER STREQUAL "none")
+        set(BUILDTYPE_LOWER "empty")
     else()
         set(BUILDTYPE_LOWER "release")
     endif()