Free param contains empty string if parking lot is closed

[kiliankoe]

If a parking lot is closed, the free parameter of said parking lot contains an empty string and not a 0. Whilst not being false, I don't think this is ok with the schema.json:

"free":
{
    "description":"Count of free parking lots",
    "type":"integer"
}

[RobTranquillo]

Ok, thx.
Some lots have a 0 some lots have just a empty field.
I will improve that,