how to render a translucent object but without BRDF

[boringfish]

Hello, everyone. I'm currently doing some experiments with path tracing rendering of transclucent materials. I want to filter out the effects of reflected light on the surface of the object (such as highlights and diffuse in BRDF), and only keep the transmission effect of light inside the object.
I tried to use only medium in the scene configuration xml file without BRDF component, but the final rendering result was only abnormal white.

I also tried to set the specular_reflectance in roughdielectric to 0 and the spec_trans in principled to 1, but I didn't get the desired effect. So, Is there a way to make the final rendering result contain only the internal scattering of the object without surface reflection? Thank you every much.
Here is my xml file：

<scene version="2.1.0">
    <path value="../../common"/>
	<default name="integrator" value="volpath"/>
	<default name="rr_depth" value="64"/>
	<default name="max_depth" value="64"/>

	<bsdf type="roughdielectric" id="object_bsdf">
        <string name="distribution" value="beckmann"/>
        <float name="alpha" value="1.0"/>
        <string name="int_ior" value="bk7"/>
        <string name="ext_ior" value="air"/>
        <rgb name="specular_reflectance" value="0.0,0.0,0.0"/>
    </bsdf>

<bsdf type="diffuse"  id="test_bsdf">
    <rgb name="reflectance" value="1.0,1.0,1.0"/>
</bsdf>

<bsdf type="principled"  id="t_bsdf">
    <rgb name="base_color" value="1.0,1.0,1.0"/>
    <float name="metallic" value="0.0" />
    <float name="eta" value="1.5" />
    <float name="roughness" value="0.05" />
    <float name="spec_tint" value="0.0" />
    <float name="anisotropic" value="0.0" />
    <float name="sheen" value="0.0" />
    <float name="sheen_tint" value="0.0" />
    <float name="clearcoat" value="0.0" />
    <float name="clearcoat_gloss" value="0.00" />
    <float name="spec_trans" value="1.0" />
</bsdf>

    <medium id="medium1" type="homogeneous">
        <float name="scale" value="2"/>

        <phase type="hg">
			<float name="g" value="0.7"/>
		</phase>

        <volume name="albedo" type="gridvolume">
            <string name="wrap_mode" value="repeat"/>
            <string name="filename" value="textures/albedo.vol"/>
        </volume>

        <boolean name="has_spectral_extinction" value="false"/>
        <boolean name="sample_emitters" value="true"/>
    </medium>

    <shape type="ply">
        <string name="filename" value="meshes/bunny.ply"/>
		<transform name="to_world">
			<scale value="13"/>
			<rotate z="1" angle="130"/>
			<rotate y="1" angle="70"/>
			<rotate x="1" angle="-50"/>
			<rotate z="1" angle="-50"/>
			<rotate y="1" angle="-2"/>
			<rotate x="1" angle="12"/>
			<translate x="0.3" y="0.0" z="-0.5"/>		
		</transform>
	<ref id="medium1" name="interior"/>
       <ref id="object_bsdf"/>
        
    </shape>

    <include filename="include/shape_scene.xml"/>

</scene>

[shinyoung-yi]

You might be looking for the null BSDF.
Here’s an example usage:

<bsdf type="null" id="object_bsdf"/>

[boringfish]

That's what I want. Thank you very much!