gooftool: allow NoneType add_file in generate report

gooftool upload_report --upload_method none will not pass add_file
Command will return TypeError in list operation.

BUG=b:147021303
TEST=manual
     gooftool upload_report --upload_method none
     gooftool upload_report --upload_method none --add_file /var/factory/log/factory.log

Change-Id: I15756bf110f61deedb75c946702bfa4533323775
Signed-off-by: Jongpil Jung <jongpil19.jung@samsung.corp-partner.google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/1980300
Reviewed-by: Pi-Hsun Shih <pihsun@chromium.org>
diff --git a/py/gooftool/commands.py b/py/gooftool/commands.py
index a788987..657561b 100755
--- a/py/gooftool/commands.py
+++ b/py/gooftool/commands.py
@@ -634,7 +634,9 @@
 
   # Intentionally ignoring dotfiles in EVENT_LOG_DIR.
   tar_cmd = 'cd %s ; tar cJf %s * -C /' % (event_log.EVENT_LOG_DIR, target_path)
-  tar_files = [paths.FACTORY_LOG_PATH, paths.DATA_TESTLOG_DIR] + add_file
+  tar_files = [paths.FACTORY_LOG_PATH, paths.DATA_TESTLOG_DIR]
+  if add_file:
+    tar_files = tar_files + add_file
   for f in tar_files:
     # Require absolute paths since we use -C / to change current directory to
     # root.