Fixed: Empty element should be merged instead of removed in deleteContents

[scofalik]

Suggested merge commit message (convention)

Fix: deleteContent algorithm will use merging to "remove" empty element which will ensure a better conflict resolution on collaborative editing. Closes ckeditor/ckeditor5#4195.

[Reinmar]

Eeeem... as you wrote this is just a workaround and it's a workaround for a collab scenario where three blocks are merged simultaneously by two users. Additionally, this scenario leads to an incorrect data, not to a crash.

Taken that, I'm against doing this change. Avoiding using the remove op may help in the meantime but will need to be reverted later on. Unless, of course, you think that such algorithms should use merge instead of remove?

[scofalik]

I proposed this PR anyway because this issue is:

• not difficult to reproduce,
• gives a very bad result,
• we won't fix it in transformations soon (requires refactor).

Also, it is not 100% clear that this should be a remove or merge. I wouldn't even say that this is a workaround - just a worse solution. But, both solutions are reasonable. I wanted to fix this in transformations, but this is impossible. So in the end, I wanted to at least fix what can be easily fixed. So we won't be stuck with a bug for a couple of months.

[Reinmar]

OK, then let's change it. I can see that the code actually got simpler and after looking at the scenario again I realised that it makes sense to use merge there.