Review follow-up. An explicit exists_fn bypasses entity._is_included's
empty-{} stub carve-out, so the sentinel-aware energy meter would drop
power_watts/energy_kwh when /device/0 returns a not-yet-fetched stub for
/energy/consumption/vs/0. Restore the carve-out (`not rep or ...`), and
hide power_watts when instantaneousPower is absent from a populated rep
(not just when it's the -500 sentinel) so a partial rep can't spawn a
phantom power sensor.