mqtt: only clear nearest/nearest_drone topic once on transition to empty

[richmobile]

This was done by Claude.

When there's no aircraft in range, publish_summary() (and publish_drone_summary())
republish an empty payload to airspace/summary/nearest (and nearest_drone) on every
tick — not just once on the transition into "empty". Since HA's MQTT integration tries
to JSON-decode every state_topic payload before running value_template, this logs a
WARNING "Erroneous JSON" + a template warning once per publish_summary_min_interval_s,
continuously, for as long as the airspace is empty (which for most installs is most of
the time).

Repro: leave airspace empty for a few minutes with debug/warning logging on the MQTT
integration. Watch homeassistant.log fill with:

WARNING (MainThread) [homeassistant.components.mqtt.entity] Erroneous JSON:
WARNING (MainThread) [homeassistant.helpers.template] Template variable warning:
'value_json' is undefined when rendering '{{ value_json.distance_to.home if
value_json else None }}'

Fix: only publish the empty-retained sentinel once, on the transition from
"has nearest" -> "no nearest", the same way _last_receiver_status already
avoids republishing unchanged receiver status every tick.