JwksTrustStore.Get returns the last fetched JWKS key (err=nil) when the requested kid is absent, instead of nil

`JwksTrustStore.Get` assigned each parsed key to the function's **named return value** while refreshing the JWKS cache. When the requested `kid` was not present, the trailing naked `return` handed the caller the last key in the JWKS document together with a `nil` error — a confidently wrong answer instead of "not found".