[CI] Fix error in setup-dev (ray-project#38342)

ray-project#38247 introduced a bug. If you --extras isn't specified, you get an error like "Can't iterate through NoneType". This PR fixes the error.

Signed-off-by: Balaji Veeramani <[email protected]>

python/ray/setup-dev.py

@@ -131,8 +131,9 @@ def do_link(package, force=False, skip_list=None, local_path=None):
  local_path="../../../dashboard",
  )
 
- for package in args.extras:
- do_link(package, force=args.yes, skip_list=args.skip)
+ if args.extras is not None:
+ for package in args.extras:
+ do_link(package, force=args.yes, skip_list=args.skip)
 
  print(
  "Created links.\n\nIf you run into issues initializing Ray, please "