Fix opentherm json

[barbudor]

Description:

Following report from Xrus on Discord : https://discord.com/channels/479389167382691863/882284950995492864/882284955445628929
tele / tasmota / SENSOR = {"Time": "2021-08-30T07: 45: 34", "SI7021": {"Temperature": 20.4, "Humidity": 76.9, "DewPoint" : 16.2}, "OPENTHERM": {"conn": "FAULT", "settings": 3,}, "Global": {"Temperature": 20.4, "Humidity": 76.9, "DewPoint": 16.2}, " TempUnit ":" C "}
Obvious useless , in the OPENTHERM section after "settings":3

Refactored the code to avoid this when no telemetry is provided

Will be tested by the user

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.1.0.7.3
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[barbudor]

@xrusteg please test using my fork https://github.com/barbudor/Tasmota/tree/fix_opentherm_json
Branch fix_opentherm_json
And report result here
Thanks

[xrusteg]

It works, json is correct:
08:48:22.448 MQT: tele/tasmota/SENSOR = {"Time":"2021-09-01T08:48:22","SI7021":{"Temperature":20.0,"Humidity":77.5,"DewPoint":15.9},"OPENTHERM":{"conn":"FAULT","settings":3},"TempUnit":"C"}