Closes #3889

There currently isn't an easy and accurate way to tell if a rendered card is empty. In rslib, a test checks for the "card is blank" substring, whereas in ankidroid:

An AnkiDroid feature that marks previewer tabs with a warning badge if they are empty is therefore forced to check for the presence of the doc page link in the rendered card text, which is kind of a hacky frontend fix

This pr proposes to modify the card rendering output to return a flag indicating if the card was empty (missing cloze or empty front), replacing the need for flaky substring matching

Breakage:

• Mobile clients
  They may need to account for the extra field in RenderCardResponse. Similar to c731c77, ankidroid might have to update https://github.com/ankidroid/Anki-Android/blob/5726c433ba269dbde5a9caa93cac0c0174c0d37b/AnkiDroid/src/main/java/com/ichi2/libanki/TemplateManager.kt#L61 to begin to make use of it
• Addons
  c731c77 changes the PartiallyRenderedCard class by adding a field. As it stands i'm not sure if this would break any existing addons, but it does not seem likely