[serialization] Deserializer should create default ImageParams (if external_params is empty)

[derek-gerstmann]

    // FIXME: We shouldn't have to populate the params ... but passing an empty map triggers an error in deserialize
    // for a missing input param
    std::map<std::string, Parameter> params; 
    ImageParam input(UInt(8), 3, "input");
    params.insert({"input", input.parameter()});

    // Now deserialize the pipeline from file
    Pipeline blur_pipeline = deserialize_pipeline("blur.hlpipe", params);

[TH3CHARLie]

I think I already had a fix for this locally, let me send a PR on Monday