Contact Form block: Make it obvious that placeholder paragraph block wouldn't redener if empty

[brbrr]

Reported by @keoshi in (internal ref) p1HpG7-5V3-p2 #comment-29006

Steps to reproduce

1. Starting at URL: Add a default Form block
2. Check whitespace between text area block and Submit button
3. there should be an empty paragraph block

What I expected

Not to be confused by whitespace between last block and Submit button. I would prefer to have a static + button to add more blocks

What happened instead

I am confused :)

Browser / OS version

Chrome / MacOS

Screenshot / Video

Context / Source