Gltf attribute attenuation_distance does not have a default value

As [seen here](https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/libraries/bxdf/gltf_pbr.mtlx#L27):
```xml

```
And if I read the [specs](https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_volume/README.md#attenuation) correctly, the default should be `+infinity`. Should we represent that using FLOAT_MAX?

Start with libraries/bxdf/gltf_pbr.mtlx at the attenuation_distance input and compare its declaration with the KHR_materials_volume attenuation specification linked in the issue. Determine how the default should represent positive infinity, then verify the updated MaterialX asset and any relevant validation or rendering behavior.

Written by the indexing model from the issue text.