possible to query a database for an empty jsonb
This is a very bad idea.

not null default '"{}"'::jsonb


That's not storing an empty object. That's storing a JSONB string literal of "{}". You never want that. You normally want to confine it to a specific subtype instead, something like this..

not null default '{}' CHECK (jsonb_typeof(unconfirmed_matrix_prices) = 'object')


You also normally don't want to make it not-null. I'd rather have nullable and prevent empty objects if I need that kind of constraint. That simplifies your query substantially (as seen here).