Set proper default input on image nodes

## Issue

For `` nodes (and nodes with functional graphs using this node), the`default` value
fallback input is not discoverable.

## Proposal

Modify the `` to use `defaultinput` instead of a hard-coded default.

## Related

This is a follow-up to the change https://github.com/AcademySoftwareFoundation/MaterialX/pull/1452 for https://github.com/AcademySoftwareFoundation/MaterialX/issues/1438
where it was decided to leave `default` input updates to the integration.

In this case the integration can determine if the texture is available and if not query the
`defaultinput` on the node's output to find the appropriate fallback texture color to use and
bind that instead.

Start by tracing how image nodes and functional graphs define their output and fallback inputs, then read the related PR 1452 and issue 1438 for the integration decision. Confirm how an integration queries defaultinput when a texture is unavailable. Done means the output uses defaultinput rather than a hard-coded default, allowing the fallback texture color to be discovered.

Written by the indexing model from the issue text.