Instant Search: Better handling of untitled posts in search results

### Impacted plugin

Jetpack, Search

### What

Posts without a title are visible in the minimal results but are inaccessible.

### How

What it looks like now:


Ideally, we'd detect this and either use a localized "Untitled" string or maybe the post slug. I'd favour the localized string because it might look odd if the site is not configured to use pretty permalinks.

Start with the Jetpack Search Instant Search entry point for minimal results and inspect how posts without titles are rendered and localized. Use the screenshot and the existing result behavior to verify the change; done means untitled posts remain accessible and display an appropriate localized label.

Written by the indexing model from the issue text.