[Minor] Fix bias in if to remove ambiguity (#3259)

vllm/model_executor/layers/linear.py

@@ -73,7 +73,7 @@ class UnquantizedLinearMethod(LinearMethodBase):
                       bias: Optional[torch.Tensor] = None) -> torch.Tensor:
         weight = weights["weight"]
         if self.separate_bias_add:
-            if bias:
+            if bias is not None:
                 return F.linear(x, weight) + bias
             return F.linear(x, weight)
         return F.linear(x, weight, bias)