Use `HdRenderPassAovBinding::clearValue` when clearing the render buffer

**Describe the feature you'd like**
Use `HdRenderPassAovBinding::clearValue` to set the default value in the render buffer.

While this is not a strict requirement when the format of the `HdRenderBuffer` and the Arnold AOV matches in component counts, when there is a discrepancy (like rendering to a float4 buffer from an RGB AOV) the lack of the proper default value can cause issues, like the alpha channel being zero.