Fixed DocumentSelection#_fixGraveyardSelection

[scofalik]

Suggested merge commit message (convention)

Fixed: DocumentSelection#_fixGraveyardSelection now does not add incorrect range when all content is removed from document, instead it is empty. Closes ckeditor/ckeditor5#4129.

[Reinmar]

Did you check all tests?

[scofalik]

I've run npm t, though I might have out-dated other repos. Is something wrong?

It looks like the build that failed had passed all the tests:
Chrome 59.0.3071 (Linux 0.0.0): Executed 3484 of 3484 SUCCESS (13.338 secs / 8.377 secs)

[Reinmar]

Is something wrong?

IDK, I just asked because this is the kind of change which might've broken something.

[scofalik]

I've run npm t and clicked a bit (but not much) in manual test.

I understand the fear, but when you think of it, the result will be for some scenarios the same, and for some clearly better, so I hope that the fix won't ruin something.