[bugfix] Fix online serving crash when text type response_format is received (#26822)

Signed-off-by: cjackal <44624812+cjackal@users.noreply.github.com>
Signed-off-by: j0shuajun <59368606+j0shuajun@users.noreply.github.com>
Co-authored-by: j0shuajun <59368606+j0shuajun@users.noreply.github.com>

vllm/sampling_params.py

@@ -67,6 +67,11 @@ class StructuredOutputsParams:
                 "You can only use one kind of structured outputs constraint "
                 f"but multiple are specified: {self.__dict__}"
             )
+        if count < 1:
+            raise ValueError(
+                "You must use one kind of structured outputs constraint "
+                f"but none are specified: {self.__dict__}"
+            )
 
     def all_constraints_none(self) -> bool:
         """