Can't pass null to Charts children

# 🐛Bug report

**Describe the bug**

I want to do this:
```jsx

{!hasDuplicateTimestamps && (

)}

```

I might have multiple events at a given timestamp. If I do, I don't want to render a line.

Most places where `children` are looped over, `null`s are checked:

https://github.com/esnet/react-timeseries-charts/blob/aa9c9b368100d78337b562d9e2833f2d90d9de3d/src/components/ChartRow.js#L312-L313

but there's no check here:

https://github.com/esnet/react-timeseries-charts/blob/aa9c9b368100d78337b562d9e2833f2d90d9de3d/src/components/ChartRow.js#L316-L317

resulting in this stack trace:

```
Uncaught (in promise) TypeError: Cannot read property 'props' of null
at ChartRow.js:377
at forEachSingleChild (react.development.js:1118)
at traverseAllChildrenImpl (react.development.js:1007)
...
```
**To Reproduce**
Steps to reproduce the behavior:
1. Pass a `null` into the `children` for a `` instance
2. View crash

**Expected behavior**
`null`s are handle. I believe it's just a matter of adding `if (chart === null) return;` to line 317 of ChartRow.js

---

want me to send a PR?