Refactor emoji fallback behavior.

!45261 (merged)

I'm leaning towards keeping the functionality as-is for the meantime. That is, retain the ability for the engineer to define whether they want to invoke the fallback emoji or not. I'm certainly not opposed to discussing this with the wider team and UX; one approach would be to solve the query is undefined bug in this MR, and open up a follow-up MR which removes the fallback param.