Open https://en.wikipedia.org/wiki/User:Neil_P._Quinn-WMF/T98930. Open the page in VisualEditor and create an empty line between "Line 1" and "Line 2". The save button will become enabled. Save the page. Check the page history. Contrary to what you'd expect, no edit has been actually been recorded.
If a change is going to be treated as null and not recorded, we shouldn't offer the user the save button.
When the user really really wants to make a null edit, we need to allow it. Despite MediaWiki not making a history entry, it does have an effect (caching, etc.). We should probably warn/alert the user though, where we can reasonably detect that it will result in a null revision (consider edit conflicts between two edits performing the same change for example, I think that might trigger it)
Null edits are not intended functionality, but they are a sad fact of life. The visual editor should have feature parity with the wikitext editors with respect to this fact of life. Maybe, one day, when we get rid of the need for people to make null edits, we can revisit this, but today is not that day.