top_p: null still sent in API requests after #385 fix, causing errors on Bedrock and other providers

[toslali-ibm]

PR #385 (closing #378) made top_p optional and changed its default to None in LLMConfig. However, the fix only addressed the config layer — the API call layer in openevolve/llm/openai.py still unconditionally includes top_p in the request params:

# openevolve/llm/openai.py ~L157
params = {
    "model": self.model,
    "messages": formatted_messages,
    "temperature": kwargs.get("temperature", self.temperature),
    "top_p": kwargs.get("top_p", self.top_p),   # <-- sends None when top_p is unset
    "max_tokens": kwargs.get("max_tokens", self.max_tokens),
}

After #385, self.top_p defaults to None, so this puts "top_p": None into the params dict. Whether this causes an error depends on the downstream API client:

• The standard OpenAI SDK may strip None values internally (uses NOT_GIVEN as its sentinel)
• AWS Bedrock, LiteLLM, and other OpenAI-compatible wrappers may serialize it as "top_p": null in the JSON body, triggering a 400 error

Reproduction

1. Configure OpenEvolve with an AWS Bedrock endpoint (or any OpenAI-compatible provider that does not strip null values)
2. Do not set top_p in the config (relying on the None default from Fix Anthropic models error when both temperature and top_p are passed #385)
3. Run an evolution — the API call includes "top_p": null, resulting in:

Error code: 400 - {'error': {'code': 'invalid_request_error', 'message': '`temperature` and `top_p` cannot both be specified for this model.'}}

Expected behavior

When top_p is None, the key should be omitted from the params dict entirely, rather than being sent as null.

Suggested fix

-            params = {
-                "model": self.model,
-                "messages": formatted_messages,
-                "temperature": kwargs.get("temperature", self.temperature),
-                "top_p": kwargs.get("top_p", self.top_p),
-                "max_tokens": kwargs.get("max_tokens", self.max_tokens),
-            }
+            params = {
+                "model": self.model,
+                "messages": formatted_messages,
+                "temperature": kwargs.get("temperature", self.temperature),
+                "max_tokens": kwargs.get("max_tokens", self.max_tokens),
+            }
+            top_p = kwargs.get("top_p", self.top_p)
+            if top_p is not None:
+                params["top_p"] = top_p