Map cmake None to meson's plain

empty is not a build type that exists in meson.

I have no idea why I typed "empty" in #3614 ...

Fixes: ba5f1d8

CMakeLists.txt

@@ -45,7 +45,7 @@ if(CMAKE_BUILD_TYPE)
     elseif(BUILDTYPE_LOWER STREQUAL "minsizerel")
         set(BUILDTYPE_LOWER "minsize")
     elseif(BUILDTYPE_LOWER STREQUAL "none")
-        set(BUILDTYPE_LOWER "empty")
+        set(BUILDTYPE_LOWER "plain")
     else()
         set(BUILDTYPE_LOWER "release")
     endif()