LinkerId: Use empty string for unknown linker id

Since commit c26c6ac488 (Link Step: compute effective linker used by the
compiler, 2023-05-27) we used `UNKNOWN` if the linker id is not known.
This convention is not used elsewhere, and logic using the linker id
is easier when `if(CMAKE_C_COMPILER_LINKER_ID)` is false on unknown id.

Modules/Internal/CMakeDetermineLinkerId.cmake

@@ -83,10 +83,6 @@ function(cmake_determine_linker_id lang linker)
       break()
     endif()
   endforeach()
-  if(NOT linker_id)
-    # unknown linker
-    set(linker_id "UNKNOWN")
-  endif()
 
   set(CMAKE_${lang}_COMPILER_LINKER_ID "${linker_id}" PARENT_SCOPE)
   if (linker_frontend)