Expose Body::empty for easier tests

[clux]

minor ergonomic improvement over Body::from(Bytes::new()). mock tests generally have to import the Body anyway

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.1%. Comparing base (362b985) to head (223ec1e).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1444   +/-   ##
=====================================
  Coverage   72.1%   72.1%           
=====================================
  Files         78      78           
  Lines       6594    6594           
=====================================
  Hits        4749    4749           
  Misses      1845    1845           

Files	Coverage Δ
kube-client/src/client/body.rs	87.9% <100.0%> (ø)