Visualization: Parent Pipeline aggregation fails when no bucket selected

[maryia-lapata]

Kibana version:
7.1.1

Elasticsearch version:
7.1.1

Describe the bug:
In visualizations, request fails when Parent Pipeline aggregation is applied without a bucket selected.

Steps to reproduce:

1. Create a visualization
2. Choose any Parent Pipeline aggregation
3. Click the Apply button

Expected behavior:
The form should be invalid, a warning message should be displayed.

Screenshots (if relevant):

Errors in browser console (if relevant):

[elasticmachine]

Pinging @elastic/kibana-app