Visualize: Upgrading vis with pipeline agg from 6.8 doesn't render

[flash1293]

Kibana version: 7.12 BC2

Describe the bug: Upgrading from 6.8 with visualizations using pipeline aggs in any way or importing visualizations from 6.8 results in rendering a blank visualization with an error in the console

Steps to reproduce:

1. Spin up 6.8 and configure a visualization using a pipeline agg (e.g. area chart with time x axis and moving avg on y axis)
2. Upgrade to 7.12 BC2
3. Open visualization
4. Empty vis rendered

Expected behavior:
Visualization should be rendered fine

Errors in browser console (if relevant):

Uncaught (in promise) Error: Objects must have a type property
    at getType (expressions.plugin.js:1)
    at getArgumentString (expressions.plugin.js:6)
    at expressions.plugin.js:6
    at Array.reduce (<anonymous>)
    at expressions.plugin.js:6
    at Array.map (<anonymous>)
    at getExpressionArgs (expressions.plugin.js:6)
    at Array.<anonymous> (expressions.plugin.js:6)
    at Array.map (<anonymous>)
    at getExpression (expressions.plugin.js:6)

Any additional context:
You can also use this export I used for testing: https://gist.github.com/flash1293/ba8a4cf238e16942d68161f1fee41541

Upgrade from 7.11 works fine AFAICT

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)