Fix initial preview

[sdirix]

What it does

Make sure preview widget content is also rendered initially for empty documents. This is important for custom preview widgets which might want to render something even if the underlying content is empty.

How to test

Create a custom preview widget handler and open it on an empty document. For example like this:

import URI from "@theia/core/lib/common/uri";
import { PreviewHandler } from "@theia/preview/lib/browser";
import { injectable } from "inversify";

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

[sdirix]

Merged upstream with eclipse-theia#8729