Goal

Fix $extensionMultiStream and $expandToDrm to return valid non-empty query shapes, and add a host-side guard that substitutes {<stageName>: {}} when any extension stage returns an empty shape. 

Context
An empty {} stage stored in the query stats representative pipeline cannot be re-parsed ("A pipeline stage specification object must contain exactly one field", 40323), so $queryStats fails on debug builds and silently drops the entry on release builds.