fix(internal): handle leading whitespace in message content

Author: blingblin-g

Diff for: src/openai/lib/streaming/chat/_completions.py

@@ -431,7 +431,7 @@ def _accumulate_chunk(self, chunk: ChatCompletionChunk) -> ParsedChatCompletionS
                         raise ContentFilterFinishReasonError()
 
             if (
-                choice_snapshot.message.content
+                choice_snapshot.message.content.strip()
                 and not choice_snapshot.message.refusal
                 and is_given(self._rich_response_format)
             ):