`CodeExpression`: `output` property not rendered when initially empty

[nokome]

This illustrates the problem - the two code expressions, which initially have no output, have successfully executed but their output is not rendered:

This is despite the fact that the output slot has been updated,

<span class="hidden slot sc-stencila-code-expression-default sc-stencila-code-expression-default-s">
  <output data-itemprop="output" slot="output">
      <span itemtype="http://schema.stenci.la/Number" itemscope="">3</span>
  </output>
</span>

When reloading the page the output is rendered properly,

Bug occurs on both Firefox and Chromium.