Restrict absorption distance to a positive value

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Editor/LitUI.cs

                         m_MaterialEditor.ShaderProperty(transmittanceColor, Styles.transmittanceColorText);
                         ++EditorGUI.indentLevel;
                         m_MaterialEditor.ShaderProperty(atDistance, Styles.atDistanceText);
+                        atDistance.floatValue = Mathf.Max(atDistance.floatValue, 0);
                         --EditorGUI.indentLevel;
 
                         --EditorGUI.indentLevel;