Publicize: refine behaviour when removing custom Publicize message

#### Steps to reproduce the issue

1. Start from a site connected to WordPress.com, where the Publicize feature is active.
2. Go to Posts > Add New
3. Add a post title and some content.
4. Save your draft.
5. Open the Jetpack plugin sidebar
6. Remove the post title that was automatically added to the custom Publicize message field
7. Hit "Save draft" again
8. The title is added back.

#### What I expected

I'm not sure what the best scenario should be here. Overall I think we'd need to rethink our default behaviour and expected result for each change in that field, as it's not consistent right now.

- We have different results depending on the type of your post (see #5384)
- Some folks would expect no message at all.
- some folks would want to see no excerpt (see #8545)

#### Related issues

- #6760
- #13297
- #261
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#### Video

https://user-images.githubusercontent.com/426388/130600237-cb23213c-8135-453e-8c94-e6c5e4617d2d.mov

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#### Old description, before this issue was reopened.

1. Using the block editor (Gutenberg), proceed to Publish a Post or Page
2. The "Customize your message" field defaults to the Post or Page's title

Image Link: https://d.pr/i/deeVdV

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#### What I expected

3. The Post or Page publishes with only the Post or Page title (or, in this case, a custom message to illustrate the point)

Image Link: https://d.pr/i/SYMZN6

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#### What happened instead

3. The "Customize your message" field is effectively ignored, resulting in Facebook using its own default message/preview

Image Link: https://d.pr/i/kvQoAl

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#### Alternatives

- Leave the "Customize your message" field blank (do not put the Post or Page title as default text)
- Allow for a default message, such as "Click to read more." (or something), so if a user leaves the field blank, the preview on Facebook is less messy

Reproduce the behavior from Posts > Add New with a WordPress.com-connected site, then inspect the Jetpack plugin sidebar’s custom Publicize message field. Read related issues #5384, #8545, #6760, #13297, and #261 before defining the expected behavior. Done means the agreed behavior is consistent when the automatically added title is removed and the draft is saved.

Written by the indexing model from the issue text.