Writing Flow: Can't forward delete from empty paragraph

[aduth]

To Reproduce

1. Navigate to Posts > Add New
2. Click writing prompt
3. Press Enter
4. Press ArrowUp
5. Press Delete (Control + Delete on Mac) for forward deletion
6. Observe nothing happens

Expected behavior

The next paragraph should collapse into the current.

[aduth]

I think this is technically a regression of #8461, itself a fix of #8388.

The logic for detecting whether the horizontal edge has been reached will now consider TinyMCE's bogus br nodes and incorrectly return false, even when there is no other content in the RichText field.