fix: don't apply extra mj-body when the parent is not mj-body

[DRoet]

This prevents mj-body logic getting added when the parent of the mj-section is actually a mj-wrapper

fixes https://github.com/artf/grapesjs-mjml/issues/150

<mj-body>
  <mj-wrapper border="1px solid #000000" padding="50px 30px">
    <mj-section border-top="1px solid #aaaaaa" border-left="1px solid #aaaaaa" border-right="1px solid #aaaaaa" padding="20px">
      <mj-column>
        <mj-image padding="0" src="https://via.placeholder.com/150" />
      </mj-column>
    </mj-section>
    <mj-section border-left="1px solid #aaaaaa" border-right="1px solid #aaaaaa" padding="20px" border-bottom="1px solid #aaaaaa">
      <mj-column border="1px solid #dddddd">
        <mj-text padding="20px"> First line of text </mj-text>
        <mj-divider border-width="1px" border-style="dashed" border-color="lightgrey" padding="0 20px" />
        <mj-text padding="20px"> Second line of text </mj-text>
      </mj-column>
    </mj-section>
  </mj-wrapper>
</mj-body>

before:

after: