t/984: Don't return the block in which selection ends if it ends at its beginning

[Reinmar]

Suggested merge commit message (convention)

Other: The Selection#getSelectedBlocks() method will not return a block in which selection ends if no content of that block is selected. Closes ckeditor/ckeditor5#4091.

For example, in the following case only the first two paragraphs will be returned:

<paragraph>[Foo</paragraph>
<paragraph>Bar</paragraph>
<paragraph>]Baz</paragraph>

The reasoning behind this change is that the user doesn't consider the last block as selected in such a case (as its selection isn't even visible).

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Additional information

Related branches:

• https://github.com/ckeditor/ckeditor5-list/tree/t/ckeditor5-engine/984
• https://github.com/ckeditor/ckeditor5-paragraph/tree/t/ckeditor5-engine/984

I'm not 100% sure that this change isn't too broad, but let's see based on users' feedback. It's simple so we can tune it up.