tools: fix make_json_list to return empty json list for empty cmake list

Currently make_json_list() returns '[ "" ]' for empty cmake list. Fix this
so empty json list is returned instead.

Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>

tools/cmake/utilities.cmake

@@ -178,7 +178,12 @@ endfunction()
 
 # Convert a CMake list to a JSON list and store it in a variable
 function(make_json_list list variable)
-    string(REPLACE ";" "\", \"" result "[ \"${list}\" ]")
+    list(LENGTH list length)
+    if(${length})
+        string(REPLACE ";" "\", \"" result "[ \"${list}\" ]")
+    else()
+        set(result "[]")
+    endif()
     set("${variable}" "${result}" PARENT_SCOPE)
 endfunction()