[API Server] arguments already have defaults, should always be present

There's no reason to use getattr() or repeat the default values for
args, since they will always be present with the default specified
in their declaration.

Fixes:

```
File "/home/markmc/vllm-project/vllm/vllm/entrypoints/openai/cli_args.py", line 338, in validate_parsed_serve_args
  if getattr(args, "api_server_count", 1) > 1:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '>' not supported between instances of 'NoneType' and 'int'
```

Signed-off-by: Mark McLoughlin <[email protected]>

Author: markmc

vllm/entrypoints/openai/cli_args.py

@@ -333,13 +333,13 @@ def validate_parsed_serve_args(args: argparse.Namespace):
     # drain shutdown only supported in single-process mode
     shutdown_mode = getattr(args, "shutdown_mode", "immediate")
     if shutdown_mode == "drain":
-        if getattr(args, "headless", False):
+        if args.headless:
             raise ValueError("--shutdown-mode=drain is not supported in headless mode")
-        if getattr(args, "api_server_count", 1) > 1:
+        if args.api_server_count is not None and args.api_server_count > 1:
             raise ValueError(
                 "--shutdown-mode=drain is not supported with --api-server-count > 1"
             )
-        if getattr(args, "data_parallel_size", 1) > 1:
+        if args.data_parallel_size > 1:
             raise ValueError(
                 "--shutdown-mode=drain is not supported with --data-parallel-size > 1"
             )