CMAKE_BUILD_TYPE initialized on Windows before CMAKE_USER_MAKE_RULES_OVERRIDE

On Linux, unless I set -DCMAKE_BUILD_TYPE=$type that variable is an empty string:

$ cat ../CMakeLists.txt
message("CMAKE_BUILD_TYPE='${CMAKE_BUILD_TYPE}'")

$ cmake -G Ninja . | grep CMAKE_BUILD_TYPE=
CMAKE_BUILD_TYPE=''

On Windows, it is Debug unless I override it explicitly:

λ cat ..\CMakeLists.txt
message("CMAKE_BUILD_TYPE='${CMAKE_BUILD_TYPE}'")

λ cmake -G Ninja . | grep CMAKE_BUILD_TYPE=
CMAKE_BUILD_TYPE='Debug'

This is a problem for us because we'd like the default to a release build, but on Windows with the Ninja generator we can't detect whether a build type has been specified explicitly or not (in every other case we can check whether CMAKE_BUILD_TYPE is empty). We see that checking CMAKE_BUILD_TYPE is not a viable approach for IDE generators, but it seems Ninja should be supported. How should we be doing this?