[Bug] AuthLayer produces JWT with "exp" claim set to null

### Component

transports

### What version of Alloy are you on?

2.0.5

### Operating System

None

### Describe the bug

AuthLayer inserts Authorization header with a JWT that is rejected as invalid by some execution clients (Besu). The reason is that the claim set in this JWT is serialized as `{"iat": , "exp": null}`: that is, instead of omitting `exp` claim, it is specified with `null` value.
Per RFC7519, §4.1.4, the value of `exp` claim must be a number.

Suggestion: set up serialization of `Claims` so that when `exp` has `None` value, it is omitted from the serialized representation.

Note also that Engine API specification does not specify the use of `exp`, it only uses `iat` for staleness detection; not sure why `exp` is included into the JWT used to authenticate Engine API.