TYPO3 Core

I've made the following xclass-override which resolves my proposal:

use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;

class RenderViewHelper extends \TYPO3Fluid\Fluid\ViewHelpers\RenderViewHelper
{
    /**
     * @return mixed
     */
    public static function renderStatic(array $arguments, \Closure $renderChildrenClosure, RenderingContextInterface $renderingContext)
    {
        $content = parent::renderStatic($arguments, $renderChildrenClosure, $renderingContext);

        if(!empty($arguments['contentAs']))
        {
            $renderingContext->getVariableProvider()->add($arguments['contentAs'], $content);

            if(($content = $renderChildrenClosure()) !== NULL)
            {
                $renderingContext->getVariableProvider()->remove($arguments['contentAs']);
            }
        }

        return $content;
    }
}

And the following patch should solve this in the original class.

This would allow such cases rendering optional wraps only if content exists:

<f:render section="Footer" contentAs="footerSection">
  <div class="footer--content">
     {footerSection -> f:format.raw()}
  </div>
</f:render>

The documentation states:

If used, renders the child content and adds it as a template variable with this name for use in the partial/section

So contentAs provides the tag content of f:render as a variable to the partial/section that should be rendered, not the other way around. Your XCLASS would introduce an infinite loop if you use the variable inside the partial as documented: You provide a variable which depends on the rendered partial to the partial itself.

As the tag content is already used for this feature, it isn't possible to implement your alternative behavior without introducing a breaking change and losing the other functionality.