Mark function_call as nullable in chat completion requests

Fixes openai/openai-python#1433

openapi.yaml

@@ -7487,6 +7487,7 @@ components:
                     type: object
                     deprecated: true
                     description: "Deprecated and replaced by `tool_calls`. The name and arguments of a function that should be called, as generated by the model."
+                    nullable: true
                     properties:
                         arguments:
                             type: string