Fping sometimes emits None messages

[vegu]

Likely when a target is unreachable.

Spotted by users when using timeseries db as output target. (see #137)

[grizz]

It is when a target is unreachable, I'm not sure what else to emit? Maybe NaN?

[vegu]

I was wondering if it should just gap by not emitting anything

[mattlan]

Shouldn't the inability to reach a target be considered 100% loss? In lieu of that perhaps just a gap as @vegu mentioned to denote that the pings were not sent/received during that cycle.

[vegu]

grizz and i discussed this some more a day or two ago and also arrived at sending a 100% loss message as being the best option.