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

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

Start by reproducing the issue from the default Form block and inspect the empty paragraph between the text area and Submit button. Compare the current whitespace with the requested static + affordance, then verify that the empty-block state is clearly understandable and that adding another block remains possible.

Written by the indexing model from the issue text.