The document with just `<script>` or `<style>` element becomes empty

[maxbarnas]

📝 Provide detailed reproduction steps (if any)

In ckeditor5-html-support/tests/manual/ghs-all-features.html:

1. Set in Source Editing mode (whitespace before or after does not matter):

<script>foo</script>

or

<style>foo</style>

2. Switch to regular mode and then back to source editing mode (or call editor.getData()).

✔️ Expected result

Same data as set.

❌ Actual result

Empty string.

[niegowski]

Seems like those model elements should be marked as isContent in the model schema (those do not contain any text, their content is stored inside a model element attribute).