[CMD_GET_BLE_ADDR] Wrong return value if list of BLE addresses was reset

[matthias-bs]

Sending (CMD_SET_BLE_ADDR) {"ble_addr": ["00:00:00:00:00:00"]} resets the list of BLE addresses in Preferences to an empty list, which lets the software apply the default list from BresserWeatherSensorLWCfg.h.

A subsequent {"cmd": "CMD_GET_BLE_ADDR"} should return this default list, instead "00:00:00:00:00:00" (which is used internally to indicate an empty list) is returned.