SceneCacheFileFormat : Create the pseudo-root spec in InitData

[jaec-ie]

Generally describe what this PR will do, and why it is needed.

• This PR fixes a USD coding error, Unsupported Spec Type for </>, that is raised when scene cache (.scc) data is added to a USD stage as a sublayer under USD 25.02 or later. The geometry still loads and draws, but the stage is left in a error state. It is found in Nuke 17 with Cortex USD, and the added unittest itself is the reproduction of this issue. If the unittest is too specific or redundant, I could drop it.

• The root cause is that UsdSceneCacheFileFormat::InitData returns a SceneCacheData with no specs, missing the pseudo-root spec that USD requires every layer's data object to contain. The fix refered to other initData in openUSD like here

Related Issues

Dependencies

Breaking Changes

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.

[murraystevenson]

LGTM. Thanks @jaec-ie!