[Bugfix] Fix qwen2.5-vl positon input bug (#639)

### What this PR does / why we need it?
Fix qwen2.5-vl positon input bug, fix #625 `TypeError: 'NoneType' object
is not iterable`

Signed-off-by: wangli <wangli858794774@gmail.com>

Author: Potabk

vllm_ascend/worker/model_runner.py

@@ -556,7 +556,8 @@ def build(self) -> ModelInputForNPU:
         #print(f"before tensor input_positions: {input_positions}")
         #print(f"before list seq_lens: {seq_lens}")
         input_tokens = flatten_2d_lists(input_tokens)
-        input_positions = flatten_2d_lists(input_positions)
+        if input_positions:
+            input_positions = flatten_2d_lists(input_positions)
         if graph_pad_size != -1 and not is_prompt:
             input_tokens.extend(itertools.repeat(0, graph_pad_size))
             input_positions.extend(  # type: ignore