llvm_tools: Add default value for dependent CLs

Use [] as a default value to avoid non-iterable type
errors.

BUG=chromium:1067029
TEST=unit tests

Change-Id: Ibc4c76c2dcda8e3846d323345aeea49ac2e9acaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2148005
Reviewed-by: George Burgess <gbiv@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
diff --git a/llvm_tools/update_packages_and_test_cq.py b/llvm_tools/update_packages_and_test_cq.py
index 17416a2..15246e4 100755
--- a/llvm_tools/update_packages_and_test_cq.py
+++ b/llvm_tools/update_packages_and_test_cq.py
@@ -53,6 +53,7 @@
   parser.add_argument(
       '--extra_change_lists',
       type=int,
+      default=[],
       nargs='+',
       help='change lists that would like to be run alongside the change list '
       'of updating the packages')