[CMake] Return empty list from 'make_paths_relative' if passed 'pathlist' is empty.

[vvereschaka]

Summary

Check passed 'pathlist' parameter and return an empty list if the 'pathlist' parameter is empty.
Otherwise the python script raises unattended error on empty parameter. The error message looks like:

Traceback (most recent call last):
 	 File "<string>", line 22, in <module>
IndexError: list index out of range

see more details here http://lab.llvm.org:8011/builders/llvm-clang-win-x-aarch64/builds/9476/steps/build-llvm-project/logs/stdio

llvm/cmake/modules/AddLLVM.cmake

 # contain symlinks are returned absolute.
 # Use with LLVM_LIT_PATH_FUNCTION below.
 function(make_paths_relative out_pathlist basedir pathlist)
+  if (NOT pathlist)
+    set(${out_pathlist} "" PARENT_SCOPE)
+    return()
+  endif()
   # Passing ARG_PATH_VALUES as-is to execute_process() makes cmake strip
   # empty list entries. So escape the ;s in the list and do the splitting
   # ourselves. cmake has no relpath function, so use Python for that.