Solis MQTT discovery publishes invalid sensor topics when inverter serial is empty

[SruiSan]

Under certain conditions, the Solis integration creates sensors with an empty inverter serial.
This results in invalid MQTT discovery topics like:

homeassistant/sensor//current_power/config

and invalid device identifiers:

"identifiers": ["solismqtt_"]

Root cause:

sensor.py creates entities without validating inverter_sn

unique_id is built using possibly empty data

Proposed fix:

Skip discovery when inverter_sn is empty

Enforce non-empty serial in SolisSensor.init

Build unique_id using {DOMAIN}{inverter_sn}{sensor_type}

This prevents invalid MQTT discovery and retained garbage topics.