small cmake string replace bug in exception-flags.cmake

This line will fail when CMAKE_CXX_FLAGS is empty string

https://github.com/simdjson/simdjson/blob/8a9daeb0ad29f2c98e34c7a3ab8252c893840cfc/cmake/exception-flags.cmake#L20

```
Exception has occurred: FATAL_ERROR
CMake Error at E:/Cache/CPM/simdjson/b60a1c5b9db0e95e26bfa614d41a391da7385d29/cmake/exception-flags.cmake:20 (string):
  string sub-command REPLACE requires at least four arguments.
```