[Document list] List items are not removed correctly with backspace

[FilipTokarski]

Part of #11377.

📝 Provide detailed reproduction steps (if any)

1. Switch to ck/10812-document-list-editing
2. Create a few list items
3. In an empty list item press backspace

✔️ Expected result

Item gets removed, cursor goes up to the previous list item.

❌ Actual result

Cursor does not go to the previous list item. All other list items get spaced out, which looks bad. Only after second backspace they get back to normal.

Screencast

https://user-images.githubusercontent.com/34380544/155957333-e7e6f6f8-15a3-40ad-ab3b-51ca38b44eaa.mp4

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[Reinmar]

Document-wise, that's an expected behavior. That's exactly what happens in Google Docs.

However, in GDoc when item 3 is merged into item 2, the item 2 is not moved "down". Additional spacing appears, but it affects item 3 only. We may want to improve styling for blocks inside lists a bit to mimic this.

[Reinmar]

That's how it works on Google Docs:

https://user-images.githubusercontent.com/156149/156347235-d3385a63-03f8-4046-a56d-25d3bc179ed9.mp4

Note, that I have paragraphs ("Normal text") style in Google Docs to have bigger space after them:

If you'd test it with the default styles that Google Docs come with (no spacing around paragraphs), it would look a bit differently:

https://user-images.githubusercontent.com/156149/156347822-bc8d6074-7375-4517-b38f-c5ad56c99f82.mp4

To sum up:

• The document changes as expected.
• We could change the styling a bit to make it less confusing.

[Reinmar]

Goals:

• Try to limit the visual noise when merging two paragraphs into one list item
• But don't overdo it – let's not overcomplicate the stylesheets

[oleq]

.ck-content li > p {
    margin: 0;
}

for starters and let's see where do we go from here.

[Witoso]

Decision

Let's apply this style to the first (margin-top) / last paragraphs (margin-bottom).

To test

Nested ol/ul, and discovery the last paragraph before next list item.

[Reinmar]

Related ticket: #16866.

[Reinmar]

Related (not functionally, but due to combined confusion that these issues create together): #16884.