API key cannot be empty in vllm_client.py

Just a minor issue but api key in vllm_client.py cannot be empty string. Any value will do but must have value.

client = openai.OpenAI(
api_key='', # must have something
base_url='http://localhost/v1',
http_client=httpx_client
)

Tested on Roihu.

Start in vllm_client.py at the openai.OpenAI client initialization shown in the issue, and reproduce the behavior against the local http://localhost/v1 endpoint. Trace how the API key is supplied and verify that the client works with an empty key while preserving authenticated usage; the relevant client behavior should be covered by a focused test if the repository provides one.

Written by the indexing model from the issue text.