Fix editor when formatting starts with a linebreak

[lahdeero]

🎩 What? Why?

Currently editor crashes when formatting start with a linebreak, for example
<strong><br>foo</strong>, or <em><br>bar</em>. This pull request fixes this and adds test.

📌 Related Issues

#7896 @alecslupu 's comment
quilljs/parchment#87

Testing

1. In console:

Decidim::Organization.first.update(admin_terms_of_use_body: { "en" => "<p>something</p><h1><br
></h1><p><strong><br></strong></p><p><u><br></u></p><p><em><br></em></p>" })

2. Go to: admin/organization/edit
3. Editor content is editable

📋 Checklist

🚨 Please review the guidelines for contributing to this repository.

• ❓ CONSIDER adding a unit test if your PR resolves an issue.
• ✔️ DO check open PR's to avoid duplicates.
• ✔️ DO keep pull requests small so they can be easily reviewed.
• ✔️ DO build locally before pushing.
• ✔️ DO make sure tests pass.
• ✔️ DO make sure any new changes are documented in docs/.
• ✔️ DO add and modify seeds if necessary.
• ✔️ DO add CHANGELOG upgrade notes if required.
• ✔️ DO add to GraphQL API if there are new public fields.
• ✔️ DO add link to MetaDecidim if it's a new feature.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.

♥️ Thank you!

[mrcasals]

Thanks, @lahdeero, for taking care of these issues! ❤️