Dashboard save: Saving incomplete field override config results in unnecessary save diff

[dprokop]

I think we should try to limit the amount of non-migration based diffs in dashboard if possible. This is one of the examples when we could avoid the Save button getting active when it actually should not:

1. Create a dashboard with a single panel.
2. Create a field override in that panel, but DO NOT SPECIFY any properties to override, only configure the matcher.
3. Save the dashboard (mind, that the incomplete override is saved).
4. Refresh dashboard page end enter edit mode, click Save dashboard (observe there are changes, incomplete override is removed)
5. Go to panel edit (observe Save dashboard is active now, incomplete override is removed).