HDRenderPipeline: Clamp value of thicknessMultiplier to avoid negative one

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

 
                         ++EditorGUI.indentLevel;
                         m_MaterialEditor.ShaderProperty(thicknessMultiplier, Styles.refractionThicknessMultiplierText);
+                        thicknessMultiplier.floatValue = Mathf.Max(thicknessMultiplier.floatValue, 0);
                         --EditorGUI.indentLevel;
 
                         m_MaterialEditor.TexturePropertySingleLine(Styles.transmittanceColorText, transmittanceColorMap, transmittanceColor);