Fix build error with newer cmake

Fixes the following build error:

CMake Error at cmake/os/Linux.cmake:29 (STRING):
STRING sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
CMakeLists.txt:101 (INCLUDE)

CMake Error at cmake/os/Linux.cmake:29 (STRING):
STRING sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
CMakeLists.txt:101 (INCLUDE)

The error happens when CMAKE_SHARED_LINKER_{LANG}_FLAGS is not set.
Force the variable to be set to "" as input to prevent this.
Signed-off-by: Ryan Coe <bluemrp9@gmail.com>
Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>