Fixes #632

Addresses the subcase in #632 where the new-block indicator wasn't shown when the block list was empty and not interacted with yet.

Also, showing the indicator when at the last block of the list.

Gutenberg PR: WordPress/gutenberg#18289

To test:

1. Locally edit the initial-html.js file to have an empty block list
2. Run the demo app
3. Tap on the "+" button to insert a new block
4. Notice the "ADD BLOCK HERE" indicator right below the title block 🎉

To test 2:

1. Locally edit the initial-html.js file to have an empty block list
2. Run the demo app
3. Type some text in the "Start writing" prompt
4. Tap on the "+" button to insert a new block
5. Notice the "ADD BLOCK HERE" indicator right below the paragraph block 🎉

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.