DPMMA-2881 fix Grapesjs-Mjml self-closing mj-spacer tag issue

[patrykgruszka]

Q	A
Bug fix? (use the a.b branch)	🟢
New feature/enhancement? (use the a.x branch)	🔴
Deprecations?	🔴
BC breaks? (use the c.x branch)	🔴
Automated tests included?	🔴
Related user documentation PR URL	mautic/user-documentation#...
Related developer documentation PR URL	mautic/developer-documentation-new#...
Issue(s) addressed	Fixes #...

Description

This pull request resolves an issue where self-closing tag <mj-spacer /> were causing rendering problems in Grapesjs-Mjml.

Fix for other self-closing tags was introduced in #13431, this PR just adds mj-spacer, to list of void tags.

---

📋 Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
   1: Go to Channels > Emails
   2: Create new
   3: Select "New template email"
   4: Fill in a name for "Subject" and "Internal name"
   5: Select the "Brienz" theme - because it uses MJML
   6: Go to "Builder"
   7: Click on the pencil icon that corresponds to "Edit code"
   8: Delete all the code and paste this code:

<mjml>
  <mj-body>
    <mj-section>
      <mj-column>
        <mj-text font-size="20px">top text</mj-text>
        <mj-spacer/>
        <mj-text font-size="20px">bottom text</mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>

10: Click save
11: Check if it's rendered correctly: top and bottom text should have a spacer element between

[tomekkowalczyk]

Change looks good ;)

[Aleks0129]

Does what it says