[Speculators][Speculative Decoding] Fix gpt-oss eagle3 accuracy issue (#25406)

Signed-off-by: jiahanc <173873397+jiahanc@users.noreply.github.com>

vllm/config/model.py

@@ -1003,6 +1003,7 @@ class ModelConfig:
                     self.quantization = quantization_override
                     break
 
+            quant_method = quant_method if quant_method != "" else None
             # Verify quantization configurations.
             if self.quantization is None:
                 self.quantization = quant_method