fix test bug in srt_llava_next_test.py

[bingwork]

otherwise it will cause error like below:

========== single ==========

/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py:328: UserWarning: Error in stream_executor: Traceback (most recent call last):
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 326, in _thread_worker_func
self._execute(expr)
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 369, in _execute
self._execute(x)
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 369, in _execute
self._execute(x)
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 375, in _execute
self._execute_image(other)
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 419, in _execute_image
base64_data = encode_image_base64(path)
File "/home/ubuntu/wubing/sglang/python/sglang/utils.py", line 116, in encode_image_base64
image.save(buffered, format="PNG")
AttributeError: 'tuple' object has no attribute 'save'

warnings.warn(f"Error in stream_executor: {get_exception_traceback()}")
Traceback (most recent call last):
File "/home/ubuntu/wubing/sglang/examples/usage/llava/srt_llava_next_test.py", line 78, in
single()
File "/home/ubuntu/wubing/sglang/examples/usage/llava/srt_llava_next_test.py", line 22, in single
print(state["answer"], "\n")
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 844, in getitem
return self.get_var(name)
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 831, in get_var
return self.stream_executor.get_var(name)
File "/home/ubuntu/wubing/sglang/python/sglang/lang/interpreter.py", line 252, in get_var
return self.variables[name]
KeyError: 'answer'