Make the landing page for families who sign up but don't immediately have a study to take very friendly

# TL;DR

Right now, a new family first sees the full set of studies, then has the disappointing experience of choosing a kid from the drop down menu and just seeing the below. We should change this, in prep for wider advertisement of the site.

# Narrative

As a brand new user, it's disappointing to not have a study to take right away! It's especially sad to see the big list of studies that disappears when I select the kid. Instead, I should get a friendly message telling me that I'll be emailed when there's a study available, and given something to click that *isn't* the old Children Helping Science platform!

# Acceptance Criteria
A precise description of how/when this issue will be considered closed, e.g.:

- [ ] Decide the new flow for new participants who haven't provided a kid with any studies yet (IE something other than the vanilla "Find a study" link we currently show)
- [ ] Draft new text (Melissa!) for the Search page that makes sense for the empty return view, while still making sense for people reading it above a list of actual studies!
- [ ] Implement changes to new-participant workflow
- [ ] Implement changes to Search page

# Implementation Notes
Leave important notes/suggestions here for implementation (important files, suggested approaches, etc.)

No files, tests, or entry points are identified in the issue, so start by locating the new-participant workflow and Search page implementation. Review the empty-study experience and the acceptance criteria; done means the flow and copy are decided, the new-participant workflow and Search page are updated, and the empty state provides a friendly email-notification message without the old platform link.

Written by the indexing model from the issue text.