Element: Apply wrapper div for RawHTML with non-children props

[aduth]

Fixes #6042

This pull request seeks to reintroduce the behavior of applying a wrapper div to RawHTML elements rendered with non-children props. This is particularly important for block serialization where the block's implementation returns only a RawHTML element, as there may be props (className) to apply to the block. This behavior had existed prior to #5897, but in moving the handling of RawHTML to native serializer support, was missed. This logic has now been moved from the component to the serializer.

Testing instructions:

Ensure unit tests pass:

npm run test-unit

Verify that when adding a columns block containing only shortcodes, the saved value includes a div wrapper for each column reflecting the column layout of the shortcode blocks.

[jorgefilipecosta]

The code looks great and I also verified in the button block that if I change save to:

<div>
      <RawHTML className="addDiv">
          { '<div>aaaa</div>' }
       </RawHTML>
</div>

In master the class was ignored and the output was:

<div class="wp-block-button">
    <div>aaaa</div>
</div>

In this branch the output is correct:

<div class="wp-block-button">
    <div class="addDiv">
        <div>aaaa</div>
    </div>
</div>

But I think we have a bug in the logic applied in the shortcodes case. If we add a shortcode without any content/input change to the second column, and we change to code view the output is:

    <!-- wp:shortcode {"layout":"column-2"} -->
    <div class="layout-column-2">undefined</div>
    <!-- /wp:shortcode --> 

[aduth]

@jorgefilipecosta Nice catch. This was noted by @bobbingwide as well, when there is no children passed to RawHTML. I wasn't sure whether to apply the fix here or in some serialization updates I had planned otherwise. I think I'll include a test case and fix here.

[aduth]

Updated in 9285350 and 2d603c9. Consolidated logic into renderNativeComponent, treating RawHTML as something of a "fragment" when it is not meant to have a wrapper.