Editor crashed in some scenarios when mutations common ancestor could not be mapped to model

[scofalik]

Suggested merge commit message (convention)

Fix: Editor crashed in some scenarios when mutations common ancestor could not be mapped to model. Closes ckeditor/ckeditor5#718.

---

Additional information

I changed VirtualTestEditor to ClassicTestEditor in tests. Mutations handling functions use DomConverter instance. However, in VirtualTestEditor, DomConverter is not used, so the tests, even though run green, were not accurate.

After the change, I removed this if: https://github.com/ckeditor/ckeditor5-typing/pull/140/files#diff-8052e4286a369903aa5d7f6c2d8b7190L190

It was covered only because the tests were using VirtualTestEditor. I cannot think of real case scenario where this could happen and all tests are green after removing it.

[coveralls]

Coverage remained the same at 100.0% when pulling 6eda231 on t/ckeditor5/718 into 430fcf7 on master.