ChartCollector Collector connection_ping_min error: min() arg is an empty sequence

I'm using `Version: 0.7.1 r4206, Python 3.7.3 ` and an error keeps coming out from logs:
`ChartCollector Collector connection_ping_min error: min() arg is an empty sequence`
This might be a result of the lack of edge condition checks in [the corresponding code](https://github.com/HelloZeroNet/ZeroNet/blob/py3/plugins/Chart/ChartCollector.py#L49
), where the built-in `min()` is sensitive to zero-length lists.