fix: renderCustomElements is not bullet-proof

[fago]

When trying to improve our default node component to use the proper slot syntax (as documented) I ran into problems with rendering custom elements erroring out.. Let's fix this by making the helper real-bullet-proof and properly documented.

It should work like this

    <slot name="image"> 
      <component :is="useDrupalCe().renderCustomElements($attrs.image)" />
    </slot>
    <slot name="body">
      <component :is="useDrupalCe().renderCustomElements($attrs.body)" />
    </slot>

However, it dies if

• the value is null, what could be fixed with a v-if but even nicer/shorter/better to have it not die and return null
• the slot is output not as a object but a simple string. apparently this happens for the body/image.

Current work-a-roudn is really not nice and not what we want to keep:

    <slot name="image"> 
      <component v-if="$attrs.image" :is="useDrupalCe().renderCustomElements({
        element: $attrs.image.element ?? 'drupal-markup',
        content: $attrs.image,
      })" />
    </slot>

Let's fix this, so renderCustomElements can be used without hassle / lots of needs the passed data is an object.