work around powershell arg parsing fail

It appears that powershell doesn't properly handle an empty argument, resulting in all the subsequent arguments being shifted left by one.

So, don't specify --optional-deps argument if it is empty.

autobuild.py

@@ -1112,6 +1112,7 @@ def write_build_plan(args: Any) -> None:
             job = job_info["matrix"]
             # TODO: pin optional deps to their exact version somehow, in case something changes
             # between this run and when the worker gets to it.
+            if optional_deps:
                 job["build-args"] = job["build-args"] + " --optional-deps " + shlex.quote(optional_deps)
             jobs.append(job)
             # XXX: If there is more than three builds we start two jobs with the second