GIF Block: Hitting return key in caption will duplicate it

### Impacted plugin

Jetpack

### Steps to Reproduce

1. Add a caption to the GIF block.
2. Hit Return.

### A clear and concise description of what you expected to happen.

I exit out of the GIF block and proceed to an empty line, like in the core Image block.

### What actually happened

The caption duplicated itself.

### Other information

https://user-images.githubusercontent.com/555336/130118289-ec8e2cc3-1ad2-4292-88e7-6052bd572e5a.mp4

### Operating System

macOS

### OS Version

_No response_

### Browser

Chrome/Chromium

### Browser Version(s)

_No response_

Reproduce the issue in Jetpack's GIF block by adding a caption and pressing Return, then compare the behavior with the core Image block. The work is done when Return exits the caption to an empty line without duplicating the caption.

Written by the indexing model from the issue text.