[BFCL] Fix API Keys Handling

[catherineruoxiwu]

The following bugs can all be reproduced from a clean, unmodified version of the repository. All of them have been addressed by this PR.

Setup 1:

• Input: test_case_ids_to_generate.json → rest: ["rest_0"]
• .env: No API key provided
• Command: bfcl generate --model gpt-4o-mini-2024-07-18 --run-ids

1. fix check_api_key_supplied implementation in utils.py. os.getenv(var) returns None instead of a string

2. After the previous issue is fixed, a NameError occurs when no result file existed and existing_ids was only conditionally defined inside a loop

   Exception has occurred: NameError  
   free variable 'existing_ids' referenced before assignment in enclosing scope
   

3. Under this setup, no test should be called, but test case rest_0 was still collected unexpectedly.
   

Setup 2:

• .env: All API keys provided
• Command: bfcl generate --model gpt-4o-mini-2024-07-18 --test-category rest

4. API keys were replaced in the rest tests JSON file, but the credential information in the question passed to the generate_result function was not correct.

[HuanzhiMao]

LGTM