[DeepSeekNextN] fix: residual of head norm can be None (#7398)

python/sglang/srt/models/deepseek_nextn.py

@@ -107,7 +107,11 @@ class DeepseekModelNextN(nn.Module):
         )
 
         if not forward_batch.forward_mode.is_idle():
-            hidden_states, _ = self.shared_head.norm(hidden_states, residual)
+            if residual is not None:
+                hidden_states, _ = self.shared_head.norm(hidden_states, residual)
+            else:
+                hidden_states = self.shared_head.norm(hidden_states)
+
         return hidden_states