autotest: Make autoserve handle empty argument

Paygen tests won't work on Skylab without it.

BUG=chromium:998671
TEST=None

Change-Id: Ice65ab3f6612f8367e902d2302939fce892f30a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1774756
Tested-by: Allen Li <ayatane@chromium.org>
Auto-Submit: Allen Li <ayatane@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Alex Zamorzaev <zamorzaev@chromium.org>
Commit-Queue: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
(cherry picked from commit 498f57f8ac959b4dc9348e9dd11f9e735ccdf0f5)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1790348
Tested-by: Alex Zamorzaev <zamorzaev@chromium.org>
diff --git a/server/autoserv_parser.py b/server/autoserv_parser.py
index f55e11e..b7dee11 100644
--- a/server/autoserv_parser.py
+++ b/server/autoserv_parser.py
@@ -250,7 +250,7 @@
         self.options, unknown_args = self.parser.parse_known_args()
         # Filter out none-positional arguments
         removed_args = []
-        while unknown_args and unknown_args[0][0] == '-':
+        while unknown_args and unknown_args[0] and unknown_args[0][0] == '-':
             removed_args.append(unknown_args.pop(0))
             # Always assume the argument has a value.
             if unknown_args: