Top loss charts misleading when there is no loss

[cnighswonger]

When there is no loss on the most recent round of probes, the loss value is set to -1 here.

If there is no loss in the probe of any target, the resulting display of smokeping.cgi?target=_charts.loss is misleading in that the charts displayed display (of course) no loss and the heading of each chart is Packets Lost -1.000000 which confusing at least.

It would seem better to either:

1. When no target shows loss display no graphs but display a message to the effect that there was no loss detected for any target.
2. At a minimum, change the heading of each chart to clearly state Packets Lost: None.

It is probably just me, but this was quiet confusing at the start, and I spent a bit of time grepping and munging with the code thinking the -1 was a bug before I discovered it was a feature.

[github-actions]

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