Preview Widget doesn't render preview for empty documents

[sdirix]

Bug Description:

Preview Widget doesn't render preview for empty documents

Steps to Reproduce:

1. Implement a new PreviewHandler which renders content for empty documents
2. Open preview on empty document
3. The preview is empty although it should render the content mentioned above

I assume this was not noticed until now as the Markdown preview is empty anyway for empty documents.

Additional information

Code

@injectable()
class MyPreviewHandler implements PreviewHandler {
  canHandle(uri: URI) {
    return 1000;
  }
  renderContent() {
    const div = document.createElement("div");
    div.innerText = "my preview";
    return div;
  }
}

Error Case

Expected Behavior