UX: show blank page placeholder instead of the blue panel that says "No Activity"

[AndrewPrigorshnev]

When looking at another user's page, if a user didn't solve any topic, we were showing this:

Now we'll be showing this instead:

One test is temporarily skipped because this change will fully apply only when I merge both this PR and https://github.com/discourse/discourse/pull/16293 in Core. I'm deliberately not pinning the plugin in the .discourse-compatibility file because merging these two PRs in any order won't lead to bugs.