feat(util): add const constructor for empty Full

[tottoto]

Adds const constructor for empty Full.

[seanmonstar]

This is what the Empty type is for, no?

[tottoto]

If client use the Empty type for the case to describe empty body where the functionality provided by Full is sufficient and do not need to prepare the own Body type, client will need to either box it and erase the type or define the own wrapping type, since Empty is a different type from Full. Being able to use an empty Full type in a const context is useful, as it allows client to define fixed responses that are often used for fallbacks.