[ML] DF Analytics creation: hitting Enter in advanced editor causes blank page

[alvarezmelissa87]

Kibana version:
latest master

Describe the bug:
Hitting Enter in the advanced editor after first removing all text causes blank page.

Steps to reproduce:

1. create new analytics job
2. enable advanced editor
3. delete all text in the json editor
4. type in a single open bracket
   
5. hit enter
6. whole page goes blank

Expected behavior:
Cursor should move to the next line in the editor.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/ml-ui (:ml)