Treat a string that only contains the formatting marker as empty for block formatting

[chipsnyder]

Fixes #2863

Related PRs

• gutenberg: [Mobile] Treat a string that only contains the formatting marker as empty for block formatting WordPress/gutenberg#27583
• WordPress-Android (For testing): [For Testing] Treat a string that only contains the formatting marker as empty for block formatting WordPress-Android#13566

To test:

Existing Block

1. Open a post with an existing Heading Block
2. Select the text and hit delete
   • Alternatively, delete each character one by one
3. Expect to see the placeholder text return after the last character is deleted and for the block to remain

New Heading Block

1. Open a post and add a new heading block
2. Type some text
   • Expect The text to be formatted like a heading block
3. Select the text and hit delete
   • Alternatively, delete each character one by one
4. Expect to see the placeholder text return after the last character is deleted and for the block to remain

Regression test cases

Taken from wordpress-mobile/AztecEditor-Android#928

Regression test cases

Heading block

1. Open the Gutenberg Editor
2. Create a Heading block containing some text
3. Press backspace until all of the text has been deleted
   Expect to see the placeholder text return after the last character is deleted

Paragraph block

1. Open the Gutenberg Editor
2. Create a Paragraph block containing some text
3. Select the full text and apply some styling like bold
4. Press backspace until all of the text has been deleted
   Expect to see the placeholder text return after the last character is deleted

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

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[antonis]

Approved with WordPress/gutenberg#27583