【bugfix】The NPU's forward_dsa_prepare_npu also needs special handling for is_nextn (#28118)

python/sglang/srt/hardware_backend/npu/modules/deepseek_v2_attention_mla_npu.py

@@ -403,9 +403,7 @@ def forward_dsa_prepare_npu(
                 latent_cache, forward_batch, k_nope, k_pe
             )
 
-    if m.skip_topk:
+    if not m.skip_topk or (m.is_nextn and prev_topk_indices is None):
-        topk_indices = prev_topk_indices
-    else:
         topk_indices = m.indexer(
             hidden_states,
             q_lora,
@@ -415,6 +413,8 @@ def forward_dsa_prepare_npu(
             layer_scatter_modes,
             dynamic_scale,
         )
+    else:
+        topk_indices = prev_topk_indices
 
     return (
         q_pe,