find_file: different results for same predefined cache variable
I just discovered a strange behavior, and I am not sure if it is on purpose or not.
The following `CMake` snippet is used:
```cmake
cmake_minimum_required(VERSION 3.1...3.21)
project(Test LANGUAGES NONE)
file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/my_file.txt" "")
find_file(MY_FILE NAMES my_file.txt PATHS "${CMAKE_CURRENT_BINARY_DIR}")
message("RESULT: ${MY_FILE}")
```
1. Evaluating this `CMakeLists.txt` display, as expected, `RESULT: .../my_file.txt`.
2. Now with command line `cmake -DMY_FILE=NOTFOUND`, we get `RESULT: NOTFOUND` rather than the expected `.../my_file.txt`.
3. And if we add type to the variable on the command line, i.e. `cmake -DMY_FILE:FILEPATH=NOTFOUND`, we get the expected result: `RESULT: .../my_file.txt`.
Is the behavior of item `2` a bug or not?
issue