[bug] Device Monitoring Breaks with ModemManager and 5G Modules

[csyost85]

The signal reporting from modem manager on 5 g modules does not always inlude rssi or rsrp, other values are used in 5g for signal reporting. Further, a user may not elect to set signal reporting, ie signal refresh rate setting blank or 0, this will also result in no signal reporting.
Openwisp monitoring is expecting rssi and rsrp to always be included in the data, if it is not, it throws an error and will not process any of the monitoring data received.
It is too severe of a consequence that a few data points that are not included would result in discarding the entire data.

[INFO 2023-08-29 15:56:52,490] module: views, process: 25409, thread: 140258370886528
Invalid data in "#/interfaces/3/mobile/signal/lte", validator says:

'rsrp' is a required property

[WARNING 2023-08-29 15:56:52,491] module: log, process: 25409, thread: 140258370886528
Bad Request: /api/v1/monitoring/device/d5be189b58164fc3832c86b51662bc5b/

[INFO 2023-08-29 15:57:09,104] module: views, process: 25409, thread: 140258370886528
Invalid data in "#/interfaces/3/mobile/signal/lte", validator says:

'rsrp' is a required property

[WARNING 2023-08-29 15:57:09,105] module: log, process: 25409, thread: 140258370886528
Bad Request: /api/v1/monitoring/device/d5be189b58164fc3832c86b51662bc5b/

[INFO 2023-08-29 15:57:16,040] module: views, process: 25409, thread: 140258370886528
Invalid data in "#/interfaces/2/mobile/signal/lte", validator says:

'rssi' is a required property

[nemesifier]

Hi @csyost85, thanks for reporting, send a patch if you can.

[mips171]

Sounds similar to #572