When true, the payload is the empty placeholder
built before the first fetch (see NodeFactory.createEmptyPointsNode).
An empty count is expected in that case, so the verbose diagnostic
and the "empty dataset" warning are suppressed — they would otherwise
fire once per points node on every scene load and look like errors.
A genuinely-empty committed dataset (default false) still warns.
Validate a LoadedPointsData payload. Logs warnings; throws only on malformed positions.