Add support for NaN to InfluxDB Publisher

[BrianJKoopman]

If a NaN is passed to InfluxDB the publisher will log an "invalid number" error. It'd be helpful to support NaN in the publisher. NaN support in InfluxDB wasn't introduced until Influx 2.0, we're still on 1.x. This thread suggests using null in the meantime. I'm not sure how this ends up looking on the Grafana end, that should be investigated when this is solved.