Fix gas cost type error

[kwridan]

• In cases where an IHD is configured for electricity meter only without a gas meter, the gas cost calculations would error

...
in <lambda>
    (js['gasmeter']['energy']['import']['day'] * js['gasmeter']['energy']['import']['price']['unitrate']), 2)
TypeError: unsupported operand type(s) for *: 'NoneType' and 'NoneType'

• This was due to the gas values being null
• We now default to 0 for any missing values

Test Plan:

• Manually run the calculation function on the following json sample

{
    "gasmeter": {
        "timestamp": "1970-01-01T00:00:00Z",
        "energy": {
            "import": {
                "cumulative": null,
                "day": null,
                "week": null,
                "month": null,
                "units": "kWh",
                "cumulativevol": null,
                "cumulativevolunits": "",
                "dayvol": null,
                "weekvol": null,
                "monthvol": null,
                "dayweekmonthvolunits": "",
                "mprn": "read pending",
                "supplier": "---",
                "price": {
                    "unitrate": null,
                    "standingcharge": null
                }
            }
        }
    }
}

[kwridan]

Would creating a standalone function to do the calculation and type checking be a bit neater perhaps?

Something along the lines of:

"func": lambda js : calculate_cost(
      js['gasmeter']['energy']['import']['price']['standingcharge'],
      js['gasmeter']['energy']['import']['day'],
      js['gasmeter']['energy']['import']['price']['unitrate']
    )

and

def calculate_cost(standing_charge, day_import, unit_rate):
  return round((standing_charge or 0) + ((day_import or 0) * (unit_rate or 0)), 2)

This can also be re-used for the electricity costs too which will probably suffer the same issue when the situation is reversed (have a gas meter, but no electricity meter).