Gutenberg: Don't save empty posts

[SergioEstevao]

Fixes wordpress-mobile/gutenberg-mobile#369

While creating this PR I noticed the following:

• Gutenberg JS doesn't send auto-save messages when the post is empty, this can cause some sync problems with the status of the Publish button ( being active when it shouldn't). Update: I proposed a fix for this issue in this GB PR: Ensure that the autosave happens on empty posts in the native apps. WordPress/gutenberg#23233, if you want to test with this active start Metro using this branch: Ensure that empty posts are not published gutenberg-mobile#2393
• I also found some issue where the message sent from GB, has the variable hasChanges set to false when there is changes, this is also confusing the status of the Navigation Publish/Update button, thinking that the post has no changes when it has changes.
• To make sure the post is never published empty I add a failsafe before the Publish action happens.

To test:

• Create a new post/page on the editor
• Write some content (title or post content)
• Check that the publish button is enabled
• Delete all content from title and content
• If the Publish button is enabled, press Publish
• Check that an alert shows up explaining that an empty post/page cannot be published
• Now add a single image block and don't write any text.
• Check that you can publish

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[etoledom]

Working good as described 🎉

Added a few comments on code. One alternative idea to check if the content is empty.
Let me know what do you think 👍

[etoledom]

As agreed on #14329 (comment), let's merge this to solve the main issue. Then we can improve the logic on a next PR.

Still let's wait for a ✅ on the text by @iamthomasbishop ?