fix(structured outputs): avoid parsing empty empty content (#2023)

Fixing #1763 where parsing often fails when content is empty string instead of None.

src/openai/lib/_parsing/_completions.py

@@ -157,7 +157,7 @@ def maybe_parse_content(
     response_format: type[ResponseFormatT] | ResponseFormatParam | NotGiven,
     message: ChatCompletionMessage | ParsedChatCompletionMessage[object],
 ) -> ResponseFormatT | None:
-    if has_rich_response_format(response_format) and message.content is not None and not message.refusal:
+    if has_rich_response_format(response_format) and message.content and not message.refusal:
         return _parse_content(response_format, message.content)
 
     return None