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.

从链接指向的 plugins/Chart/ChartCollector.py 第 49 行附近开始，跟踪 connection-ping 数据如何到达 min() 调用。复现或检查空数据路径，然后验证在没有可用值时 collector 不再发出此错误。

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