[Lens] App Leave Confirmation is Always Triggered

[ThomThomson]

Kibana version: master, 7.9

Describe the bug: When editing a lens visualization, leaving even without making any edits, will trigger the app leave confirmation.

Steps to reproduce:

1. Load any existing lens visualization from the visualize listing page
2. Try to reload the page, or exit via the Visualize breadcrumb

Expected behavior: The app leave confirmation should only be triggered

Screenshots (if relevant):
Looking at a dif between lastKnownDoc?.expression and state.persistedDoc.expression, it seems like the x and y titles aren't properly loaded into the lastKnownDoc

[elasticmachine]

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

[flash1293]

Thanks for reporting! @wylieconlon looks like this is the expression-incompatibility I mentioned in the meeting we already have in there because of the axis options PR.

It should be fixed with #74523 (as the expression won't be part of the saved object anymore)

[wylieconlon]

Fixed by #74523, just tested