Silence CMake warning caused by PROJECT_ROOT_DIR (#47015)

Summary:
Pull Request resolved: #47015

CMake is complaining that this variable is set outside the build but not used.
I'm using it inside our top level .cmake file to silence this warning.
We need this variable as user projects might use it.

Changelog:
[Internal] [Changed] - Silence CMake warning caused by `PROJECT_ROOT_DIR`

Reviewed By: javache

Differential Revision: D64334462

fbshipit-source-id: 558bfc9f647e62890e3cc25628b5544880c47e4b

packages/react-native/ReactAndroid/cmake-utils/ReactNative-application.cmake

@@ -35,6 +35,11 @@ if(CMAKE_HOST_WIN32)
         string(REPLACE "\\" "/" BUILD_DIR ${BUILD_DIR})
 endif()
 
+if (PROJECT_ROOT_DIR)
+# This empty `if` is just to silence a CMake warning and make sure the `PROJECT_ROOT_DIR`
+# variable is defined if user need to access it.
+endif ()
+
 file(GLOB input_SRC CONFIGURE_DEPENDS
         *.cpp
         ${BUILD_DIR}/generated/autolinking/src/main/jni/*.cpp)