I appreciate the design to always uphold the invariants of every type.
For jwt::jwks, which is an object holding a collection of jwks, constructing an empty collection of jwks is valid and maintains the invariants of the type.
The motivation for this is encapsulating jwt::jwks in a caching class, where JWKs are often cached with periodic refreshing. There aren't any keys until fetched from issuer. I found myself doing jwt::jwks{"{\"keys\":[]}"} since there wasn't a default constructor, which feels silly.