We already do this in the update pass. That's what
`shouldMeasureSuspenseNode` does.
We also don't update measurements when we're inside an offscreen tree.
However, we didn't check if the boundary itself was in a suspended state
when in the `measureUnchangedSuspenseNodesRecursively` path.
This caused boundaries to disappear when their fallback didn't have a
rect (including their timeline entries).
