Fix illegal tokens during sampling (#676)

docs/sampling_params.md

@@ -7,7 +7,7 @@ The `/generate` endpoint accepts the following arguments in the JSON format.
 @dataclass
 class GenerateReqInput:
     # The input prompt. It can be a single prompt or a batch of prompts.
-    text: Union[List[str], str]
+    text: Optional[Union[List[str], str]] = None
     # The token ids for text; one can either specify text or input_ids.
     input_ids: Optional[Union[List[List[int]], List[int]]] = None
     # The image input. It can be a file name, a url, or base64 encoded string.