MRML panics when using mj-divider inside mj-hero

[LorenzoLeonardini]

Similar to #453, but in this case the expected result is for this to work:

<mjml>
  <mj-body>
    <mj-hero>
      <mj-divider />
    </mj-hero>
  </mj-body>
</mjml>

mj-hero should behave like a mj-section with a single mj-column.

thread '<unnamed>' panicked at /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/mrml-4.0.1/src/mj_divider/render.rs:34:61:
called `Option::unwrap()` on a `None` value

my stacktrace is a bit useless because it comes from mjml-python, and I don't really know rust to reproduce it directly :')

[jdrouet]

Thanks, I'll have a look 😉