Description

• fixes the error when CMAKE_CXX_FLAGS is empty.
• related to issue small cmake string replace bug in exception-flags.cmake #2510 .

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation / tests
• Other (please describe):

How to test

> mkdir build
> cd build
> cmake -G "Visual Studio 17 2022" -A x64 -DSIMDJSON_DEVELOPER_MODE=OFF -DCMAKE_CXX_FLAGS="" ..

After fix there's no error.