VictoryArea/VictoryLine should support graphing with gaps in the y data

# Bugs and Questions

### Checklist

* [x] This is not a `victory-native` specific issue. (Issues that only appear in `victory-native` should be opened [here](https://github.com/FormidableLabs/victory-native/issues/new))

* [x] I have read through the [FAQ](https://formidable.com/open-source/victory/docs/faq) and [Guides](https://formidable.com/open-source/victory/guides/) before asking a question

* [x] I am using the latest version of Victory

* [x] I've searched open issues to make sure I'm not opening a duplicate issue

### The Problem

I'm trying to support time ranges of data, and sometimes this data has sparse values but I want the chart to make the data actually render across the whole time range. When I pass data with null values for days in-between value days, the graph will not render for either VictoryArea or VictoryLine graphs with a custom theme

Here's an example of the type of data I want to render:
const data = [

{ x: '10/21/2018', y0: 59, y: 60 },
{ x: '10/22/2018', y0: null, y: null },
{ x: '10/23/2018', y0: 2, y: 3 },
{ x: '10/24/2018', y0: null, y: null },
];

### Reproduction
Graph with interim null values
https://codesandbox.io/s/xo818yl3yz

If you remove the custom stroke color in the theme it works..
https://codesandbox.io/s/qly4kxv726