USD Look Export displacement with split components connection

[julsVFX]

Version: Gaffer 1.16.50.0-linux
Third-party tools: Arnold
Third-party modules: /

Description

When exporting a USD Look with ArnoldDisplacement, the displacement will not work if the input is a split rgb component (see attached screenshot)

Steps to reproduce

1. Create an aiNoise, split the output rgb components and connect only r into the ArnoldDisplacement input
2. Export as USD look and reassign
3. --> Displacement is not working

Debug log

Click to Expand

[15:21:04] [15:21:38] Coding Error: in New at line 54 of /__w/dependencies/dependencies/USD/working/OpenUSD-24.08/pxr/usd/sdf/attributeSpec.cpp -- Cannot create attribute spec on </main/materials/material_7d2aa04d71a58f220c627b848b6f3a08/arnold_displacement_shaders/aiCache_displacement_grey_ZBrush_a> with invalid name 'outputs:out.r'

[johnhaddon]

Related to #4055. The issue here is that the aiNoise is the last shader in the network, so IECoreScene::addComponentConnectionAdapters() doesn't see a connection to insert into. But fixing #4055 will likely entail adding a new terminal shader, such that there would be a connection.