Empty inline element should be removed when selection leaves it.

[Artur Delura]

Example use case

1. Open editor and clear it's content.
2. Type some text: abcdefg
3. Put caret here: ab^cdefg
4. Select font size: 72
5. Put caret somewhere else: abcd^efg
6. Select font size: 24

As you can see cursor fit 72 but we don't have any 72 text. That's because empty span hasn't been removed.

Since it's affect only visual part of editor (empty elements are removed on save). We might consider using CSS3 :empty pseudo selector:

span:empty {
  display: none;
}

But it's more workaround rather sultion, and it's not fully supported by browsers and it's not bulletproof.

[Piotrek Koszuliński]

cc

[Jakub Ś]

This issue is related to #12684.

We should remove empty inline elements in non-empty lines while we could/should leave empty inline elements in empty lines.