fix(requestor): make params None if it's empty dict to prevent replac…

…ement (httpx #3433)

httpx issue: When passing params={}, always strictly update rather than merge with an existing querystring.

PR: encode/httpx#3364
Issue: encode/httpx#3433

src/handyllm/requestor.py

@@ -251,7 +251,7 @@ def _call_raw(self) -> requests.Response:
             data=self.data,
             json=self.json_data,
             files=self.files,
-            params=self.params,
+            params=self.params or None,
             stream=self._stream,
             timeout=self.timeout,
         )
@@ -367,7 +367,7 @@ async def _acall_raw(self):
             data=self.data,
             json=self.json_data,
             files=self.files,
-            params=self.params,
+            params=self.params or None,
             timeout=self.timeout,
         )
         response = await self._async_client.send(