fix: supply a valid axisconfig position default (#32335) (#32362)

The previous default was an empty string, but empty strings are not valid in the expression. The code ends up using "left" or "top" (depending on the axis), so defaulting to either isn't really a change.

This fixes the following valid expression:

```
demodata | pointseries x=@timestamp y="mean(price)" | plot yaxis={axisConfig max=120}
```

Previously this would fail because the `position` default is invalid unless you specified a value.

<img width="617" alt="screenshot 2019-03-01 12 11 47" src="https://user-images.githubusercontent.com/404731/53660475-33104a80-3c1b-11e9-9a4c-a93116598e28.png">

Now it works as expected:

<img width="536" alt="screenshot 2019-03-01 12 11 03" src="https://user-images.githubusercontent.com/404731/53660440-212ea780-3c1b-11e9-8f4f-7d83a1009fd0.png">

x-pack/plugins/canvas/canvas_plugin_src/functions/common/axisConfig.js

@@ -23,7 +23,7 @@ export const axisConfig = () => ({
       types: ['string'],
       help: 'Position of the axis labels - top, bottom, left, and right',
       options: ['top', 'bottom', 'left', 'right'],
-      default: '',
+      default: 'left',
     },
     min: {
       types: ['number', 'date', 'string', 'null'],